Child entities are now deleted by an event handler of their service
class.

* 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.

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/ref...

Renamed UserAuthentication to ClientAuthentication and moved it to v2.
API v3 uses its own model.

Moved PI timer logic to its own service. The implementation for room
owner changes has been removed for now.

Moved logic operating on multiple repositories to service layer.

Repositories now implement Spring Data interfaces. Basic CouchDB
operations are now implemented once by `CouchDbCrudRepository` instead
of having redundant methods to handle each design document.

* Prefer ids over "foreign" domain objects.
* Consistently use final keyword for implementation parameters and
  variables.
* As a consequence, session id is used instead of the object as cache
  key for `skillquestions`, `lecturequestions`, `preparationquestions`
  and`flashcardquestions` caches.
* Rename occurances of `questionId` to `contentId`.

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