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

add property to enable export to click



See merge request !31

Free text validation is no longer done for abstentions.

This fixes a regression introduced in
9fc360d6.

Clear caches at fixed time intervals



See merge request !30

Real-time updates for global statistics are unnecessary and put a lot of
pressure on the database. Data is now cached for five minutes. The count
of online users is still updated in real-time.

import / export freetext questions



See merge request !29

This simplifies analysis of monitoring data.