Jackson cannot instantiate non-static inner classes for deserialization.

Use `authProvider` + `loginId` instead of `username`.

Since VisitedRoom does not extend Entity, @JsonProperty has to be set
explicitly for id.

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

A new service, repository and entity have been introduced to handle file
attachments to other entities. Attachments store meta data about the
file. The file upload is not yet implemented. It is planned to abstract
the storage handling to allow the support for different storage
solutions (local file system, database, cloud).

Additionally moved previously missed v2 entities to the v2 package.

Introduced content groups which replace question variants and support
sorting.

Limit logging for profile handling to WARN level for privacy reasons.