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

The entity is now stored in a separate property.

Additionally, roomId is used instead of Room object for legacy
RoomEvents.

* 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...

Additionally, added AnswerController for API v3.