[ZEPPELIN-6312] Remove configuration WS usage in frontend

[seung-00]

What is this PR for?

This PR removes usage of the configuration API via WebSocket in the frontend.
The configuration API exists in both WS and REST, but unlike other APIs, it doesn’t seem to need two separate channels.
Maintaining both surfaces increases maintenance cost, so this PR unifies them by keeping only the REST API.

What type of PR is it?

Improvement

Todos

• - ZEPPELIN-6313 Drop configuration WS endpoint in backend

What is the Jira issue?

• ZEPPELIN-6312

How should this be tested?

• Check the configuration page (/configuration)
• Check the notebook page (/notebook/{notebook_id})

Screenshots (if appropriate)

Questions:

• Does the license files need to update? M
• Is there breaking changes for older versions? N
• Does this needs documentation? N

[tbonelee]

Replacing WS with REST API seems reasonable since it reduce complexity.
As the login page is currently not working properly, that issue should be fixed as well.

[seung-00]

I’ve updated according to the review comments.
But I found one issue: the configurations WS API has no auth check, while the REST API does. So users without permission can still access the configs via WS.

Removing WS right now would break things, so I’ll fix this in another PR before merging this one.

@tbonelee

[tbonelee]

Could you also take a look at this? It seems that fetching configurations in `ConfigurationService.initialize()` when the user is not logged in might be causing some issues.