[ZEPPELIN-1085] Make dropdown menu scrollbar always visible
[ZEPPELIN-1085] Make dropdown menu scrollbar always visible

### What is this PR for?
In Zeppelin docs website, there are 4 dropdown menus(Quick Start, Interpreters, Display System and More) in the navbar. Especially **Interpreters** and **More** menu have lots of lists so I made this dropdown menu scrollable in #1004.
Originally the scroll bar is shown up when  a user is trying to scroll down. That's why it's hard to notice there are more menus in the dropdown menu.  See [this issue](https://issues.apache.org/jira/browse/ZEPPELIN-1085).

So I made this scroll bar always visible. But only for chrome & safari. Unfortunately, Firefox doesn't support custom scroll bar. See [here](http://stackoverflow.com/questions/18317634/force-visible-scrollbar-in-firefox-on-mac-os-x).
In short, it says
>There is no way to actually "force" Firefox render the old-style scrollbar since the default scrollbar used in the system is predefined by the OS itself (note that you can modify which scrollbar you want in System Preferences).

### What type of PR is it?
Improvement

### TODO
- [x] : test this in major browsers

### What is the Jira issue?
[ZEPPELIN-1085](https://issues.apache.org/jira/browse/ZEPPELIN-1085)

### How should this be tested?
[Build the Zeppelin docs website](https://github.com/apache/zeppelin/tree/master/docs#build-documentation) in your local and check the dropdown menu in navbar. The scrollbae has to be shown right after you click **Interpreters** or **More** menu.

### Screenshots (if appropriate)
 - Before : scrollbar is only shown up when you are trying to scroll up & down
![before](https://cloud.githubusercontent.com/assets/10060731/16647507/8c6fbc60-446a-11e6-9a57-b00a76e6b59c.gif)

 - After : always visible
![after](https://cloud.githubusercontent.com/assets/10060731/16647513/901e4e3a-446a-11e6-840b-599b6121fe46.gif)

 - In case of IE, you can always see the scrollbar. So it's not a problem originally.
![screen shot 2016-07-07 at 5 41 27 pm](https://cloud.githubusercontent.com/assets/10060731/16647584/e3708ff8-446a-11e6-8aee-c03eaba70cc9.png)

### Questions:
* Does the licenses files need update? no
* Is there breaking changes for older versions? no
* Does this needs documentation? no

Author: AhyoungRyu <fbdkdud93@hanmail.net>

Closes #1143 from AhyoungRyu/ZEPPELIN-1085 and squashes the following commits:

4d10883 [AhyoungRyu] Make dropdown menu scrollbar always visible

prabhjyotsingh/incubator-zeppelin,onkarshedge/incubator-zeppelin,joroKr21/incubator-zeppelin,lresende/incubator-zeppelin,mfelgamal/zeppelin,vipul1409/zeppelin,apache/zeppelin,datalayer/zeppelin-datalayer,sravan-s/zeppelin,sravan-s/zeppelin,zjffdu/zeppelin,apache/incubator-zeppelin,openthings/zeppelin,VipinRathor/zeppelin,doanduyhai/incubator-zeppelin,IceKhan13/zeppelin,IceKhan13/zeppelin,fogbeam/zeppelin_mirror,1ambda/zeppelin,mwpenny/zeppelin-esri,vipul1409/zeppelin,r-kamath/zeppelin,jongyoul/incubator-zeppelin,pravin-dsilva/zeppelin,minahlee/incubator-zeppelin,YuanGunGun/zeppelin,soralee/zeppelin,zetaris/zeppelin,astroshim/incubator-zeppelin,prabhjyotsingh/zeppelin,vgmartinez/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,tinkoff-dwh/zeppelin,mwkang/zeppelin,mfelgamal/zeppelin,prabhjyotsingh/incubator-zeppelin,vgmartinez/incubator-zeppelin,anthonycorbacho/incubator-zeppelin,cquptEthan/incubator-zeppelin,raysteam/zeppelin,doanduyhai/incubator-zeppelin,tillrohrmann/incubator-zeppelin,mfelgamal/zeppelin,joroKr21/incubator-zeppelin,jongyoul/incubator-zeppelin,yu74n/zeppelin,Leemoonsoo/zeppelin,onkarshedge/incubator-zeppelin,astroshim/incubator-zeppelin,optimizely/incubator-zeppelin,zjffdu/zeppelin,1ambda/zeppelin,SarunasG/zeppelin-oidc,zjffdu/zeppelin,r-kamath/zeppelin,volumeint/zeppelin,chilang/zeppelin,radicalbit/incubator-zeppelin,onkarshedge/incubator-zeppelin,namanmishra91/zeppelin,sohaibiftikhar/zeppelin,sergeymazin/zeppelin,Leemoonsoo/incubator-zeppelin,cfries/zeppelin,r-kamath/incubator-zeppelin,wary/zeppelin,mwpenny/zeppelin-esri,yu74n/zeppelin,tinkoff-dwh/zeppelin,sctincman/zeppelin,VipinRathor/zeppelin,r-kamath/zeppelin,SarunasG/zeppelin-oidc,1ambda/zeppelin,spacewalkman/incubator-zeppelin,doanduyhai/incubator-zeppelin,openthings/zeppelin,mwkang/zeppelin,tillrohrmann/incubator-zeppelin,volumeint/zeppelin,datalayer/zeppelin,tzolov/incubator-zeppelin,tinkoff-dwh/zeppelin,raysteam/zeppelin,datalayer/zeppelin,cfries/zeppelin,Madhuka/incubator-zeppelin,r-kamath/incubator-zeppelin,mfelgamal/zeppelin,vgmartinez/incubator-zeppelin,Altiscale/incubator-zeppelin,namanmishra91/zeppelin,chilang/zeppelin,optimizely/incubator-zeppelin,anthonycorbacho/incubator-zeppelin,apache/incubator-zeppelin,openthings/zeppelin,spacewalkman/incubator-zeppelin,rconline/zeppelin,hammertank/zeppelin,benoyantony/zeppelin,zetaris/zeppelin,prabhjyotsingh/incubator-zeppelin,pellmont/zeppelin,sergeymazin/zeppelin,cquptEthan/incubator-zeppelin,vrlo/zeppelin,soralee/zeppelin,lresende/incubator-zeppelin,herval/zeppelin,tzolov/incubator-zeppelin,benoyantony/zeppelin,doanduyhai/incubator-zeppelin,Madhuka/incubator-zeppelin,sravan-s/zeppelin,jongyoul/zeppelin,jongyoul/incubator-zeppelin,karuppayya/zeppelin,raysteam/zeppelin,apache/zeppelin,minahlee/incubator-zeppelin,cfries/zeppelin,karuppayya/zeppelin,sctincman/zeppelin,herval/zeppelin,pravin-dsilva/zeppelin,radicalbit/incubator-zeppelin,YuanGunGun/zeppelin,jongyoul/zeppelin,hammertank/zeppelin,yorek/zeppelin,yu74n/zeppelin,zjffdu/zeppelin,datalayer/zeppelin,prabhjyotsingh/incubator-zeppelin,lresende/incubator-zeppelin,jongyoul/zeppelin,vgmartinez/incubator-zeppelin,prabhjyotsingh/zeppelin,fogbeam/zeppelin_mirror,Leemoonsoo/incubator-zeppelin,Leemoonsoo/zeppelin,optimizely/incubator-zeppelin,jongyoul/incubator-zeppelin,zetaris/zeppelin,benoyantony/zeppelin,mwpenny/zeppelin-esri,karuppayya/zeppelin,wary/zeppelin,namanmishra91/zeppelin,vrlo/zeppelin,optimizely/incubator-zeppelin,r-kamath/zeppelin,minahlee/zeppelin,chilang/zeppelin,radicalbit/incubator-zeppelin,SarunasG/zeppelin-oidc,vipul1409/zeppelin,minahlee/incubator-zeppelin,tzolov/incubator-zeppelin,vipul1409/zeppelin,namanmishra91/zeppelin,yu74n/zeppelin,raysteam/zeppelin,jongyoul/incubator-zeppelin,joroKr21/incubator-zeppelin,sergeymazin/zeppelin,pravin-dsilva/zeppelin,apache/incubator-zeppelin,hkropp/incubator-zeppelin,datalayer/zeppelin,tinkoff-dwh/zeppelin,sohaibiftikhar/zeppelin,AlexanderShoshin/zeppelin,minahlee/zeppelin,Altiscale/incubator-zeppelin,datalayer/zeppelin-datalayer,rconline/zeppelin,astroshim/zeppelin,zetaris/zeppelin,sravan-s/zeppelin,Leemoonsoo/zeppelin,IceKhan13/zeppelin,astroshim/zeppelin,vipul1409/zeppelin,tinkoff-dwh/zeppelin,sctincman/zeppelin,fogbeam/zeppelin_mirror,SachinJanani/zeppelin,r-kamath/incubator-zeppelin,vgmartinez/incubator-zeppelin,tillrohrmann/incubator-zeppelin,cfries/zeppelin,volumeint/zeppelin,zjffdu/zeppelin,Leemoonsoo/zeppelin,prabhjyotsingh/zeppelin,datalayer/zeppelin,onkarshedge/incubator-zeppelin,tinkoff-dwh/zeppelin,IceKhan13/zeppelin,digitalreasoning/incubator-zeppelin,rconline/zeppelin,hkropp/incubator-zeppelin,pellmont/zeppelin,karuppayya/zeppelin,Altiscale/incubator-zeppelin,sergeymazin/zeppelin,mwkang/zeppelin,mwkang/zeppelin,prabhjyotsingh/zeppelin,Nova-Boy/zeppelin,mfelgamal/zeppelin,SachinJanani/zeppelin,Madhuka/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,chilang/zeppelin,sergeymazin/zeppelin,hammertank/zeppelin,minahlee/incubator-zeppelin,apache/zeppelin,mfelgamal/zeppel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in,fogbeam/zeppelin_mirror,sctincman/zeppelin,mwpenny/zeppelin-esri,SarunasG/zeppelin-oidc,vrlo/zeppelin,joroKr21/incubator-zeppelin,minahlee/zeppelin,YuanGunGun/zeppelin,lresende/incubator-zeppelin,spacewalkman/incubator-zeppelin,VipinRathor/zeppelin,herval/zeppelin,spacewalkman/incubator-zeppelin,pravin-dsilva/zeppelin,minahlee/zeppelin,Nova-Boy/zeppelin,pravin-dsilva/zeppelin,jongyoul/zeppelin,Nova-Boy/zeppelin,yu74n/zeppelin,rconline/zeppelin,volumeint/zeppelin,joroKr21/incubator-zeppelin,datalayer/zeppelin,pellmont/zeppelin,Leemoonsoo/zeppelin,joroKr21/incubator-zeppelin,SarunasG/zeppelin-oidc,radicalbit/incubator-zeppelin,hammertank/zeppelin,Leemoonsoo/incubator-zeppelin,soralee/zeppelin,SachinJanani/zeppelin,volumeint/zeppelin,hkropp/incubator-zeppelin