Revisions are copied by default but explicitly skipped for
V2ToV3Migration.

Using format caused problems because Jackson does not support multiple
type names being mapped to a single class.

This reverts commit 96b0cf42.

For now, only Answer and AnswerStatistics migrations are tested.

While type works fine for deserialization from CouchDB, it is not part
of the public v3 API. The format property is available in both cases.

A new cache 'room.id-by-shortid' has been introduced to speed up ID
lookups.

Some legacy identifiers remain very persistently.

Additionally, remove obsolete VisitedRoom repository and renamed
VisitedRoom references to RoomHistory.

Additionally, remove obsolete MotdListRepository.

Implemented migration of settings.

Implemented a customized WithMockUser and
WithMockUserSecurityContextFactory to mock an Authentication with User
and UserProfile.

Improve architecture to better fit in with Spring Security:
* Create an unauthenticated token first and pass it to an
  AuthenticationProvider.
* Let Spring Security handle everything else.

Correctly wire up UserService and set the stored, encrypted password so
it is available for comparison.

* 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'