Unfortunately, Spring Security cannot easily be used with our WebSocket
implementation since connections share a Thread and therefore share a
SecurityContext. An AuthenticationToken for WebSocket access to the
service layer is created.

Added new service for comments.

* 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`

(Lecturer/Skill)Question has been renamed to Content. Method names have
not been touched yet to ease reviewing the changes.

InterposedQuestion has been renamed to Comment. Method names have not
been touched yet to ease reviewing the changes.

Additionally, removed unused empty constructor

Additionally, remove unused loggers.

* Remove unused code
* Optimize imports (group 10+ and static imports)
* Remove redundant variable assignments
* Replace explicit type parameters with <>
* Remove redundant catch blocks
* Use varargs instead of explicit array instantiation
* Weaken visibility of methods and fields
* Simplify conditions
* Do not use StringBuilder for simple concatenations
* Fix code style
* Remove invalid Javadoc

These events add more detail about what specific
action just happened.

Unlock is a better representation because this event only
occurs when multiple questions are published. 'New' should
only be used when questions are created.

Check user object against null before accepting feedback via Websocket.

Task #15666
Task #15693

Just for consistency.