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.

remove sort order code since it's not used and imperformant



See merge request !33

Flashcard revisited

Backend changes for flashcard variant. See arsnova/arsnova-mobile!39

See merge request !32