Content groups are now a separate entity with their own repository and
service and endpoint. A content group can no longer be set when creating
a new content. Instead the new API endpoint has to be used to add the
content to a group.

This fixes get for multiple IDs and find.

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).