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

The cleanup is now disabled by default and can be set up by uncommenting
the configuration parameter `session.guest-session.cleanup-days`.

This reverts commit 3e969181.

Conflicts:
	src/main/java/de/thm/arsnova/controller/LecturerQuestionController.java

fix bug where abstention questions are not exported

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

refactored handling of logo during the saving of a new session and the update of an existing session out into a new method handleLogo

Retrieved @Cacheable and changed updateSession method in SessionService, now getting the session by keyword and set all fields that may have changed to a new value

This also fixes a test error.