There is no request parameter name set, so forwarding will fail. Fixed
by this patch.

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

getInterposedQuestions for .../{questionId} path and renamed method to
singular form.

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

Removed sessionKey from routes if it is not needed.
Changed sessionKey from PathVariable to RequestParam if still needed.
/session/{sessionKey}/question/* => /question/byaudience/*
Added redirections for legacy routes to LegacyController.

This allows expecting those exceptions in test classes.

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

QuestionByLecturerController.