Moved PI timer logic to its own service. The implementation for room
owner changes has been removed for now.

Additionally moved previously missed v2 entities to the v2 package.

With the use of JSON views they are no longer necessary.

Moved logic operating on multiple repositories to service layer.

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.

This improves/fixes compatibility with third-party OAuth APIs.

Consistently prioritize the `api.path` setting before falling back to
`servletContext.getContextPath()`.

Add messages to exceptions and slightly refactor exception handling.

Exceptions are now always passed to the logger. The logger decides based
on configuration whether and how they are logged. Logging messages were
adjusted for consistency.

Execption messages in API responses is disabled by default because they
can contain sensitive data. The config property
`api.expose-exception-messages` has been added and is described in the
developer documentation.

Error responses now expose exception details.

Exeption handling was broken by
56fe085e (GH-27).

Additionally, removed unused empty constructor

Additionally, remove unused loggers.

Changed `security.facebook.enabled-roles` to
`security.facebook.allowed-roles`.

* 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