Merge branch 'swagger-api' into 'master'

Interactive API documentation

Supersedes !5.

> This feature supports developers with an interactive documentation of
> the REST API from the ARSnova Backend.

Changes & Todos:
* Branch & code clean up
  * Rebase on master
  * Squash related commits authored by the same developer
  * Remove commits whose code has been reverted later on
  * Fix whitespaces
* Remove implementation notes (Java signatures are not helpful)
* Improve documentation of responses (Tom Käsler)
  * Do not define status messages for methods individually (DRY)
* Create JSON at compile-time by using
  [swagger-maven-plugin](https://github.com/kongchen/swagger-maven-plugin) (Tom Käsler)

See merge request !10