Rename functions to be better understandable

remove unneccessary imports
use correct Nullable from checkerframework

Fix feedback messaging

See merge request arsnova/arsnova-backend!121

The version set for Pac4j was previously ineffective because it was set
by spring-security-pac4j first.

The Spring Platform BOM has reached EOL, so we need to replace it for
future updates. This should not change any dependency versions.

https://spring.io/blog/2018/04/09/spring-io-platform-end-of-life-announcement

See GH-65.

Add websocket handler to component scan

See merge request arsnova/arsnova-backend!119

add WebSockets with STOMP on top

See merge request arsnova/arsnova-backend!112

remove unused imports

move ws handler to websocket package

add WebSocketMessage entity structure
add basic feedback logic
add tests for Feedback messages

These tests exercise the happy path. If needed, negative tests could
be added in the future.

Ektorp's default implementation expects purge_seq to be of type int.
CouchDB 2.x purge_seq is of type String.

Reverts 8ad7cd84

This fixes issue #33. However, it is still unclear why different
beans are shared between test and production code.

Both the RoomRepository as well as the CacheManager beans were
not shared between test and production classes. Hence, when we
set up some mocks (or caches), the mocked values were not
present in the production classes.

We explicitly force instance sharing by applying the Singleton pattern.

* Update MathJax URL to use version 2.7.5
* Piwik was renamed to Matomo