Release version 2.5.0

Major features:
* Evaluation of free text answers
* The flashcards UI has been overhauled. They are now handled separately
  from questions.
* Question duplication and import from other sessions

Minor features and changes:
* The number of comments is now shown in presentation mode
* New use case including only comments
* Export of answer statistics to CSV format
* Export of questions to arsnova.click format
* Export/import of flashcards to/from arsnova.cards format
* Proxy support for WebSocket connections
* Reduced the amount of API requests sent
* Usability improvements and bug fixes

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.