Refactor event handling

See merge request arsnova/arsnova-backend!100

@EventListener is used for handler methods instead of implementing
a visitor interface.

Refactor services and repositories

See merge request arsnova/arsnova-backend!99

* Prefer simple IDs over entity types for method parameters
* Avoid use of obsolete v2 classes in service and repository layers
* Optimize imports

The format for answers is now set based on the content's format.

fix comparison

See merge request arsnova/arsnova-backend!97

Fix possible null pointer dereference

See merge request !98

Fixes regression introduced by commit
c88cce27.

fix exporting all answers on session export

See merge request !93

Tom Käsler authored 6 years ago and Daniel Gerhardt committed 6 years ago

See merge request !94

Migration version now falls back to -1 if no document exists.

This fixes a logging issue with threading causing messed up log
messages.

Logback

See merge request arsnova/arsnova-backend!91

A log file path can now be set via `arsnova.log.file` Java system
property. If a log file path is set, the threshold for STDOUT logging is
increased to WARN.