Add 'soft' deletes for OAI-PMH

[netsensei]

OAI-PMH models the concept of deleted records. These are records that are no longer available in the repository, but are still listed in the endpoint. When a client harvests a record which is denoted as "deleted", it can take an appropriate action based on this status.

Currently, the Datahub does not support this. Using DELETE on a record, will perform a hard removal of the resource without any trace.

This issue involves:

• Research if SoftDeletable is a good starting point to do the heavy lifting at the database facing side.
• The REST controller should still return a 404 when requesting a soft deleted entity.
• The OAI-PMH controller needs to include records which were soft deleted, expressed according to the OAI-PHM model (use proper XML elements)

Tangential, this means that:

• All existing tests should still remain green.
• Add functional tests for OAI-PMH deletes.