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* CouchDB4J has been replaced by Ektorp.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

Mosts of the tests were coupled with the old database layer.
Additionally, a lot of them were testing simple CRUD operations intead
of domain logic. Meaningful tests with proper database mocking have to
be written at a later time.

Previously, the dependency was indirectly included via CouchDB4j.

Code for the following domains has been migrated:
* VisitedSession
* MotdList
* Statistics
* LearingProgress

(Lecturer/Skill)Question has been renamed to Content. Method names have
not been touched yet to ease reviewing the changes.

InterposedQuestion has been renamed to Comment. Method names have not
been touched yet to ease reviewing the changes.

LoggedIn documents are also handled by the SessionRepository for now.

Java's ScriptEnging is used to transform design doc JS objects into
valid JSON. A custom CouchDbConnector has been implemented to handle
this transformation and create the design docs in the database at
startup.

Priortizing v3 caused problems for v2 clients on POST requests.

This decouples type identifiers used in CouchDB from Java type names.

The creation of new documents is blocked if an empty _rev is set.

Ektorp is introduced as a replacement for CouchDB4J.

Functions need to be stringified to create valid JSON which is accepted
by CouchDB.

Additionally, removed obsolete TODO comment.

The use of a proxy is no longer necessary with AspectJ.

AspectJ fixes

See merge request !67

Use AspectJ weaving instead of JDK/cglib proxies for AOP

See merge request !66

Daniel Gerhardt authored 7 years ago and Daniel Gerhardt committed 7 years ago

This eliminates issues in edge cases like calling annotated methods from
inside the same class. The use of `AopContext.currentProxy()` and the
`exposeProxy` setting are no longer necessary for these cases.

Add missing annotations to reenable scheduling

See merge request !65

Scheduling was disabled unintentionally with the config migration in
commit 2ecbfec8.

Annotations are added as a replacement for <task:annotation-driven />.