Variable für Schwellwert zur richtigen Beantwortung einer Planquadratfrage im Backend implementiert.

CouchDBDao und Question-Entity um neue Attribute fuer Toggle Anzahl Markierungen aufheben erweitert.

These checks will be done by service and security implementation if needed

This affects:
* Some null checks before instanceof checks
* CouchDBDao.collectUnansweredQuestionIds()
* DeprecatedApiInterceptorHandler.getDeprecatedApiAnnotation()
* CacheControlInterceptorHandler.getCacheControlAnnotation()
* CacheControlInterceptorHandler.setCacheControlResponseHeader()
* FeedbackService - userService private field

Had to fix CouchDBDao.getSessionFromId() to implement working permission
evaluation.

This small method had thrown net.sf.json.JSONException wich causes
Spring to send HTTP 405 - Method not allowed and block request.

Permission handling is the same as with removed Authorized annotation

Note that this is currently untested. :o

Introduced a new CouchDB view method, and a new field to divide
'skill_questions' into preparation, lecture, and flashcard
variants. The former question type 'flashcard' will be removed
soon.

Lots of redundand code deleted thanks to this change

Feedback is stored in memory on the server, and not inside
CouchDB any more.