@EventListener is used for handler methods instead of implementing
a visitor interface.

* Prefer simple IDs over entity types for method parameters
* Avoid use of obsolete v2 classes in service and repository layers
* Optimize imports

Consistently use singular for packages which contain mixed kinds of
classes. Moved a few classes to packages that better fit them.

This was causing issues with aliases containing dots.

Fixes regression introduced by commit
7eec95ae.

Spring Platform has been upgraded to the "Cairo" release.
This introduces new framework/library versions including:
* Spring Framework 5.0
* Spring Security 5.0
* Spring Integration 5.0
* Spring Data "Kay"
* Jackson 2.9

The following code changes were made as part of the migration:
* CouchDbCrudRepository uses new method names of CrudRepository.
  An extended version of CrudRepository is used to ease the migration
  process.
* AppConfig implements WebMvcConfigurer instead of extending
  WebMvcConfigurerAdapter.
* Servlet Filter and Maven exclusions for logging have been removed.

References:
https://spring.io/blog/2017/09/28/spring-framework-5-0-goes-ga
https://github.com/spring-projects/spring-framework/wiki/What%27s-New-in-Spring-Framework-5.x
https://github.com/spring-projects/spring-framework/wiki/Upgrading-to-Spring-Framework-5.x
https://docs.spring.io/spring-framework/docs/5.0.x/spring-framework-reference/
https://docs.spring.io/spring-security/site/docs/5.0.x/reference/html/new.html
https://spring.io/blog/2017/10/02/spring-data-release-train-kay-goes-ga
https://docs.spring.io/spring-data/data-commons/docs/2.0.x/reference/html/
https://github.com/FasterXML/jackson/wiki/Jackson-Release-2.9

The Java default of handling .properties files as ISO 8859-1 encoded is
not contemporary anymore.

Default to 'vnd.de.thm.arsnova.v3+json' if no specific content type is
requested via Accept header or path version prefix.

PathApiVersionContentNegotiationStrategy selects the media type based on
request path. It allows to set the correct media type for old clients
which do not set the 'Accept' header.

* Moved classes from `de.thm.arsnova` root package.
* New util package
* Do not start interface names with `I`. Appended `Impl` to the
  implementing class instead.
* Renamed learning progress classes to score and move them from
  `.domain` to a new `.services.score` package.
* Do not capitalize acronyms in class names:
    * CASLogoutSuccessHandler -> CasLogoutSuccessHandler
    * SocketIO* -> Socketio*
* Renamed `.socket` package to `.websocket`

Fixes a regression introduced by commit
2ecbfec8.

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.

Ektorp is introduced as a replacement for CouchDB4J.

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.

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

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

API v3:
* Media type: application/vnd.de.thm.arsnova.v3+json
* Serializes date as strings conforming to ISO 8601.

API v2:
* Media type: application/vnd.de.thm.arsnova.v2+json
* Serializes dates as timestamps including nanoseconds.

An indentation setting has been added: `api.indent-response-body:false`.