Add an endpoint for deleting all objects related to an entity

[joelvdavies]

Dependent on #28 and #38. This endpoint should be a DELETE at /entity/<entity_id> that:

• Deletes all attachments related to the entity
• Deletes all images related to the entity

Will need consideration on how to deal with errors mid way, I expect they can be mitigated by deleting the database documents after successful deletion from the object store like we already do for the others.