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