Isolate the database entities to avoid accidental over exposure on WebAPI

[AKlaus]

Currently, the database entities (aka "collections") are defined in "Domain" project (along with consumer facing DTOs) that may lead to accidental overexposure of some DB data objects (fully or partially) on the API.

Acceptance criteria

• Isolate the database models in a separate project (called "Database") that is NOT included in the API project
• The new "Database" project
  • It's concern is closely coupled with RavenDB - has defined entitles and aggregates along with indexes and the way to connect to the DB;
  • it can be referenced in other projects but as a private reference only (to avoid reference propagation);
• All shared classes need to go another project