UserService instead of a CouchDB request.

UserService.
Added a scheduled method to delete inactive users from
user2sessionLegacy Map.

broadcasting it in reportSessionDataToClient method.

CouchDB and visited sessions.

Socket.IO.

requested.

implementation updates.

interfere with the existing HTTP polling solution.

Some features still need to be ported:
- update session attribute lastOwnerActivity in CouchDB
- add session to visitedSessions in CouchDB
- check if session is locked

This reverts commit 6445fea1.

This reverts commit b2d929f8.

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

API specification.

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

If the current session is linked to a course and there are other sessions
for this course, add a number to the name and short name of the session,
indicating which session is ment since all sessions will have the same
name and short name.