Lists of visited sessions for guest users which have not been active for
a fixed amount of time are now automatically deleted.

fix bug where abstention questions are not exported

implement: via two booleans you can now decide whether AnswerStatistics and FeedbackQuestions shall be exported, too

Added few pp Attributes to SessionInfo. Requests for Public Pool Sessions now return SessionInfo instead of Session.

Updated copyright year and adjusted header format to be consistent over
all files and with other ARSnova projects.

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