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

Jetty 9.3 increases the JRE requirement to version 8.

Tests failed because they did not allow a charset parameter in
Content-Type headers.

Admin interface

This merge request implements api functionality for the admin interface (https://git.thm.de/arsnova/arsnova-admininterface).

See merge request !23