[ZEPPELIN-2924] Fix broken javascript source map
[ZEPPELIN-2924] Fix broken javascript source map

### What is this PR for?

Fixed broken javascript source map. The coverage loader should be used only for test env.

**that's really annoying because we can't set breakpoint and even can't see the original source code.**

### What type of PR is it?
[Bug Fix]

### What is the Jira issue?
[ZEPPELIN-2924](https://issues.apache.org/jira/browse/ZEPPELIN-2924)

### How should this be tested?

1. run the webapp as dev mode: `localhost:9000`
2. open the developer console and click any log message
3. check whether source is minified or not.
4. execute `yarn run test` in the terminal to check that coverage feature works well.

### Screenshots (if appropriate)

#### Before

![image](https://user-images.githubusercontent.com/4968473/30260684-cc095240-9703-11e7-9fd6-fe5bd370a6bd.png)

![image](https://user-images.githubusercontent.com/4968473/30260690-d4f3494c-9703-11e7-854b-29d6020eaa3c.png)

#### After

![image](https://user-images.githubusercontent.com/4968473/30260654-a27579d6-9703-11e7-9d2a-a37c7ae7b148.png)

![image](https://user-images.githubusercontent.com/4968473/30260655-a9c62802-9703-11e7-9d29-be10df0302d7.png)

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

Author: 1ambda <1amb4a@gmail.com>

Closes #2580 from 1ambda/ZEPPELIN-2924/fix-source-map-is-broken and squashes the following commits:

375e13945 [1ambda] fix: Use coverage loader only for test env

apache/zeppelin,datalayer/zeppelin-datalayer,herval/zeppelin,datalayer/zeppelin,zjffdu/zeppelin,sravan-s/zeppelin,joroKr21/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,Leemoonsoo/zeppelin,yu74n/zeppelin,zetaris/zeppelin,sravan-s/zeppelin,doanduyhai/incubator-zeppelin,pellmont/zeppelin,herval/zeppelin,prabhjyotsingh/zeppelin,tinkoff-dwh/zeppelin,datalayer/zeppelin,karuppayya/zeppelin,joroKr21/incubator-zeppelin,jongyoul/zeppelin,sravan-s/zeppelin,vipul1409/zeppelin,yu74n/zeppelin,datalayer/zeppelin-datalayer,apache/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,r-kamath/incubator-zeppelin,jongyoul/zeppelin,zjffdu/zeppelin,doanduyhai/incubator-zeppelin,doanduyhai/incubator-zeppelin,sravan-s/zeppelin,herval/zeppelin,cquptEthan/incubator-zeppelin,namanmishra91/zeppelin,apache/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,yu74n/zeppelin,hammertank/zeppelin,astroshim/zeppelin,VipinRathor/zeppelin,astroshim/zeppelin,karuppayya/zeppelin,pellmont/zeppelin,apache/zeppelin,jongyoul/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,namanmishra91/zeppelin,herval/zeppelin,apache/zeppelin,datalayer/zeppelin,vipul1409/zeppelin,anthonycorbacho/incubator-zeppelin,prabhjyotsingh/zeppelin,prabhjyotsingh/zeppelin,vipul1409/zeppelin,sravan-s/zeppelin,astroshim/zeppelin,r-kamath/zeppelin,tzolov/incubator-zeppelin,jongyoul/incubator-zeppelin,lresende/incubator-zeppelin,datalayer/zeppelin,hammertank/zeppelin,karuppayya/zeppelin,tinkoff-dwh/zeppelin,zetaris/zeppelin,r-kamath/incubator-zeppelin,datalayer/zeppelin,sravan-s/zeppelin,astroshim/zeppelin,fogbeam/zeppelin_mirror,herval/zeppelin,anthonycorbacho/incubator-zeppelin,Leemoonsoo/zeppelin,apache/incubator-zeppelin,cquptEthan/incubator-zeppelin,apache/zeppelin,lresende/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,tzolov/incubator-zeppelin,prabhjyotsingh/zeppelin,yu74n/zeppelin,datalayer/zeppelin-datalayer,mwkang/zeppelin,joroKr21/incubator-zeppelin,r-kamath/incubator-zeppelin,VipinRathor/zeppelin,r-kamath/zeppelin,joroKr21/incubator-zeppelin,r-kamath/zeppelin,astroshim/incubator-zeppelin,joroKr21/incubator-zeppelin,mwkang/zeppelin,astroshim/zeppelin,doanduyhai/incubator-zeppelin,VipinRathor/zeppelin,tinkoff-dwh/zeppelin,karuppayya/zeppelin,apache/incubator-zeppelin,datalayer/zeppelin-datalayer,anthonycorbacho/incubator-zeppelin,zjffdu/zeppelin,tzolov/incubator-zeppelin,hammertank/zeppelin,pellmont/zeppelin,jongyoul/zeppelin,r-kamath/zeppelin,jongyoul/incubator-zeppelin,cquptEthan/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,herval/zeppelin,cquptEthan/incubator-zeppelin,r-kamath/incubator-zeppelin,apache/zeppelin,prabhjyotsingh/incubator-zeppelin,r-kamath/zeppelin,apache/zeppelin,jongyoul/zeppelin,Leemoonsoo/incubator-zeppelin,hammertank/zeppelin,doanduyhai/incubator-zeppelin,jongyoul/zeppelin,vipul1409/zeppelin,astroshim/incubator-zeppelin,lresende/incubator-zeppelin,cquptEthan/incubator-zeppelin,zjffdu/zeppelin,Leemoonsoo/zeppelin,prabhjyotsingh/incubator-zeppelin,herval/zeppelin,zetaris/zeppelin,pellmont/zeppelin,tinkoff-dwh/zeppelin,vipul1409/zeppelin,fogbeam/zeppelin_mirror,tinkoff-dwh/zeppelin,sergeymazin/zeppelin,r-kamath/incubator-zeppelin,Leemoonsoo/zeppelin,datalayer/zeppelin,r-kamath/zeppelin,doanduyhai/incubator-zeppelin,fogbeam/zeppelin_mirror,joroKr21/incubator-zeppelin,karuppayya/zeppelin,prabhjyotsingh/zeppelin,prabhjyotsingh/incubator-zeppelin,anthonycorbacho/incubator-zeppelin,astroshim/incubator-zeppelin,fogbeam/zeppelin_mirror,jongyoul/zeppelin,apache/incubator-zeppelin,mwkang/zeppelin,vipul1409/zeppelin,zetaris/zeppelin,joroKr21/incubator-zeppelin,zjffdu/zeppelin,VipinRathor/zeppelin,sergeymazin/zeppelin,pellmont/zeppelin,prabhjyotsingh/zeppelin,astroshim/incubator-zeppelin,Leemoonsoo/incubator-zeppelin,anthonycorbacho/incubator-zeppelin,astroshim/zeppelin,zetaris/zeppelin,sergeymazin/zeppelin,cquptEthan/incubator-zeppelin,prabhjyotsingh/zeppelin,r-kamath/zeppelin,lresende/incubator-zeppelin,anthonycorbacho/incubator-zeppelin,tzolov/incubator-zeppelin,doanduyhai/incubator-zeppelin,namanmishra91/zeppelin,datalayer/zeppelin,jongyoul/incubator-zeppelin,hammertank/zeppelin,VipinRathor/zeppelin,joroKr21/incubator-zeppelin,Leemoonsoo/zeppelin,astroshim/incubator-zeppelin,sergeymazin/zeppelin,anthonycorbacho/incubator-zeppelin,fogbeam/zeppelin_mirror,Leemoonsoo/incubator-zeppelin,apache/incubator-zeppelin,namanmishra91/zeppelin,astroshim/incubator-zeppelin,karuppayya/zeppelin,vipul1409/zeppelin,r-kamath/incubator-zeppelin,jongyoul/incubator-zeppelin,sergeymazin/zeppelin,mwkang/zeppelin,lresende/incubator-zeppelin,Leemoonsoo/zeppelin,mwkang/zeppelin,namanmishra91/zeppelin,sravan-s/zeppelin,lresende/incubator-zeppelin,sergeymazin/zeppelin,yu74n/zeppelin,hammertank/zeppelin,hammertank/zeppelin,pellmont/zeppelin,namanmishra91/zeppelin,pellmont/zeppelin,jongyoul/incubator-zeppelin,Leemoonsoo/zeppelin,apache/zeppelin,yu74n/zeppelin,mwkang/zeppelin,fogbeam/zeppelin_mirror,sergeymazin/zeppelin,r-kamath/incubator-zeppelin,Leemoonsoo/zeppelin,astroshim/zeppelin,zetaris/zeppelin,fogbeam/zeppelin_mirror,yu74n/zeppelin,lresende/incubator-zeppelin,tinkoff-dwh/zeppelin,tinkoff-dwh/zeppelin,zetaris/zeppelin,namanmishra91/zeppelin,karuppayya/zeppelin,jongyoul/zeppelin,VipinRathor/zeppelin,zjffdu/zeppelin,cquptEthan/incubator-zeppelin,prabhjyotsingh/incubator-zeppelin,mwkang/zeppelin,tzolov/incubator-zeppelin,tzolov/incubator-zeppelin