This upgrades Mockito to version 2.x.

Upgraded to Spring Boot BOM version 2.1.

Missing dependency versions from legacy spring io platform bom are added
as properties.

Fixes regression introduced by commit
c88cce27.

This fixes a logging issue with threading causing messed up log
messages.

Support for java system properties to tune logging have been added:
* arsnova.log.exceptions
* arsnova.log.level
* arsnova.log.level.root
* arsnova.log.level.spring
* arsnova.log.properties

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/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

* AuthenticationProvider-agnostic User object allows access to common
  user attributes: userId, loginId, authProvider, etc.
* Auto-create UserProfiles for external accounts
* Use userId (instead of loginId) for permission checks
* Use custom implementation for Pac4j integration
  (remove org.pac4j.spring-security-pac4j)
* Move authentication logic from controller to service layer
* Remove user room role handling
* Rename targetDomainType 'session' to 'room'