Move deletion of session related data to DAO method

See merge request !40

Questions and answers of affected sessions are now deleted.

Delete inactive guest visitedsessions

Depends on arsnova/arsnova-setuptool!9

See merge request !39

Lists of visited sessions for guest users which have not been active for
a fixed amount of time are now automatically deleted.

The cleanup is now disabled by default and can be set up by uncommenting
the configuration parameter `session.guest-session.cleanup-days`.

remove annotation for http servlet response



See merge request !38

Fix missing validation for MotDs

The backend wrongly trusted user input allowing the circumvention of
access control checks.

See merge request !37

This reverts commit 3e969181.

This reverts commit 5f5699c5.

The backend wrongly trusted user input allowing the circumvention of
access control checks.

Delete inactive guest sessions

Sessions of guest users which have not been visited by the owner for a
fixed amount of time are now automatically deleted.

Depends on arsnova/arsnova-setuptool!8

See merge request !36

Delete inactive users in intervals

Users which have not been activated are now automatically deleted after a fixed amount of time.

Depends on arsnova/arsnova-setuptool!7

See merge request !34

Add flashcard count to /statistics

Refs arsnova/arsnova#34

See merge request !35

The new version is required for upcoming changes.

The JavaMail API w/o implementation was accidentally used with an
upgrade of the Maven dependency.

This fixes a regression introduced in commit
0b0204e9.