[ZEPPELIN-2924] Fix broken javascript source map

[1ambda]

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

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

After

Questions:

• Does the licenses files need update? - NO
• Is there breaking changes for older versions? - NO
• Does this needs documentation? - NO

[jongyoul]

LGTM

[1ambda]

thanks for review. merge if no more discussion