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.

question. See commit 990eb9fe.

**/lecturerquestion/{questionId}/answer/.

API specification.
Changed StatisticsController to allow requests without a trailing slash
to be consistent with the other **/statistics paths.

/questionbylecturer/* => [/session/{sessionKey}]/lecturerquestion/*
/questionbyaudience/* => [/session/{sessionKey}]/audiencequestion/*

This allows expecting those exceptions in test classes.

implementation yet!).

This prevents crashes caused by null pointers. Checks are moved to
the service class, and real objects are passed along.

Removed sessionKey from routes if it is not needed.
Changed sessionKey from PathVariable to RequestParam if still needed.
/session/{sessionKey}/question/* => /question/bylecturer/*

QuestionByLecturerController.