Refactor auth handling

Large parts of auth handling have been refactored to improve maintainability and performance:

Room access control

• Room role mappings are no longer cached in localStorage.
• New API endpoints are used to retrieve room memberships and room summaries (basic info + stats). Instead of querying data for each room individually they are now fetched in bulk.
• Membership and summary data is only loaded once and cached in-memory until the cache is invalidated though a client-side or WebSocket (implementation not final / server-side implementation missing) event.
• Room authorization handling was moved to a new RoomMembershipService.
• Account management methods were moved to user service.
• Authorization header is only overridden if it wasn't already set.
• UserRole is now a string-based enum (code constructs like user.role < 3 are no longer allowed). Templates can now use the string and now longer need to hard-code a number.
• The AuthenticationGuard now returns an Observable.
• A Resolver is used to pass the role for viewing to components.
• Use helper methods in components to check the role (instead of component fields).

App-wide authentication handling

• ClientAuthentication is now used throught the app and replaces the User class. Previously, it was only used to handle the API response. User was basically the same as ClientAuthentication with a few additional properties which became obsolete properties which became obsolete.
• AuthenticationService's getUser, getUserAsSubject and watchUser methods have been replace with a single getAuthentication method which returns an Observable<ClientAuthentication>. Internally, a higher-order Observable is used to emit changes to authentication as a stream.
• The logic of AuthenticationService's login-related methods have been refactored. Instead of returning the strings true, false and activation, a ClientAuthenticationResult is return which contains the status and the auth object if successful.
• refreshLogin logic has been refactored to reuse code for normal logins.
• An APP_INITIALIZER is used to initialize authentication at startup.

Known issue

• Role-based, conditional rendering for elements of the header bar no longer works (e.g. session ID in comment list for room owner).