Implement OIDC authentication UI

[chrismatix]

Prerequisite: #1518

With OIDC the authentication state will be managed entirely by Spring as an HTTP session. This means that the frontend no longer has to store/handle a JWT for authentication. However, the inbox UI still needs to be aware of who is currently authenticated or not.

To address this problem #1518 adds a /users.getPrincipal endpoint that will return the profile information of the currently logged-in users.

So the flow would be as follows:

1. User loads the react app
2. App requests /client.config before rendering anything
3. If principal present: Store the user information in redux (not local storage) and proceed
4. If 403 unauthorized: Redirect to oidc auth endpoint (see docs provided in Add OIDC authentication middleware #1518)

If the oidc flow is successful the backend will redirect the user back to the app after 4. and the process will repeat.

[chrismatix]

@lucapette I think an even simpler implementation of this would be to move the inbox UI to the communication pod already. That way accessing the frontend itself is subject to authentication and the frontend does not need to implement any re-directs.

[chrismatix]

And since this would mean that we would replace Lua with (most likely) freemarker we can also pre-render in the user information so that the UI does not have to make a separate request to /users.getProfile