Merge branch 'db-ektorp' into 'master'

Migration to Ektorp and refactoring of the database layer

A summary of the changes:
* CouchDB4J has been replaced by Ektorp.
* CouchDBDao has been split up into multiple repositories: One for each
  entity.
* Jackson is used instead of JSON-lib for POJO <-> JSON
  (de)serialization.
    * Jackson's `@JsonView`'s are used to mark which properties are
      (de)serialized for the API and/or CouchDB
    * A lot of persistence code could be simplified since serialization
      is handled automatically by the `ObjectMapper`.
* Entities and related Classes have been renamed:
    * `Content` <- `Question` / `SkillQuestion` / `LecturerQuestion`
    * `Comment` <- `FeedbackQuestion` / `AudienceQuestion`
* CouchDB design docs can now be automatically created by the backend if
  necessary. They are included as JavaScript code which is parsed and
  transformed to JSON at startup using Java's `ScriptEngine`.
* `MediaType`s for API versioning have been introduced:
    * `vnd.de.thm.arsnova.v2+json`: the format as used in ARSnova 2.x
    * `vnd.de.thm.arsnova.v3+json`:
        * ISO 8601 dates instead of timestamps
        * `id` instead of `_id`, `revision` instead of `_rev`
* `api.indent-response-body` can be set in configuration for pretty
  responses for API debugging.
* Empty (e.g. `null`) entity properties are ignored for serialization

See merge request !68