ZEPPELIN-2984: allow enter from username
ZEPPELIN-2984: allow enter from username

### What is this PR for?
Allow an "Enter" keypress in the username field during login

### What type of PR is it?
Improvement

### Todos
* N/A

### What is the Jira issue?
* https://issues.apache.org/jira/browse/ZEPPELIN-2984

### How should this be tested?
* Pull up login screen. Fill in username and password. Click in username field.
  Press enter. We would expect the login handler to be called.
* Pull up login screen. Fill in username and password. Press enter (while still
  in password field). We would expect the login handler to be called just as
  before.
* Alternatively, use a password manager (e.g. LastPass). Save the password.
  Pull up the login screen. Press Enter. We would expect the login handler to be
  called.

### Screenshots (if appropriate)

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

Author: Gabe Fernando <gef756@gmail.com>

Closes #2616 from gef756/dev-login-enter and squashes the following commits:

301d398 [Gabe Fernando] MRG: Merge remote-tracking branch 'origin/master' into dev-login-enter
2a1865a [Gabe Fernando] (ZEPPELIN-2984) ENH: allow enter from username

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