Implemented a customized WithMockUser and
WithMockUserSecurityContextFactory to mock an Authentication with User
and UserProfile.

* AuthenticationProvider-agnostic User object allows access to common
  user attributes: userId, loginId, authProvider, etc.
* Auto-create UserProfiles for external accounts
* Use userId (instead of loginId) for permission checks
* Use custom implementation for Pac4j integration
  (remove org.pac4j.spring-security-pac4j)
* Move authentication logic from controller to service layer
* Remove user room role handling
* Rename targetDomainType 'session' to 'room'

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

* Organize imports
* Add missing Override annotations
* Remove trailing whitespace

Renamed TEACHER to SPEAKER to adapt roles to ARSnova client, and
also added some convenience methods for role handling.

All relevant information about users (e.g. session mebership) are
accessible via UserService.
The method IUserService::getCurrentUser provides information about
the actual logged in user. If no user is logged in, this method will
throw a UnauthorizedException which will end up in 401 HTTP error.

This checks if there is a user in current security context. If not, an
UnauthorizedException is thrown which will result in HTTP - 401.