Release version 2.5.0

Major features:
* Administration API: New endpoints have been added which are accessible
  by users defined by `security.admin-accounts`.
* Evaluation of free text answers
* Proxy support for WebSocket connections: It is now possible to use the
  same port for standard HTTP requests and WebSocket connections.
  Additionally, it is no longer necessary to setup a Java key store for
  TLS if a proxy is used.
* Auto-deletion of inactive (not activated) users and guest sessions

Minor features and changes:
* Caching improvements
* New use case including only comments
* Export of questions to arsnova.click format
* Export/import of flashcards to/from arsnova.cards format
* Flashcards are now handled separately from questions

Configuration changes:
* `socketio.ip` has been replaced by `socketio.bind-address`
* `security.ssl` has been removed. `security.keystore` and
  `security.storepass` have been replaced by `socketio.ssl.jks-file` and
  `socketio.ssl.jks-password`.
* New setting: `socketio.proxy-path`
* The default port for WebSocket connections has been changed to `8090`

With this release we have completely overhauled our
[documentation](README.md). Additionally, we now provide
[Docker images](https://github.com/thm-projects/arsnova-docker/).
This tag has no release notes.