Consider cascading review queues on deletes
Followup from !489 (comment 491743434):
As we always delete the related entry from the queue right after we delete the manifest, should we add the
(repository_id, manifest_id)
as anON DELETE CASCADE
FK so that it is deleted if the referenced manifest is deleted? It would remove the manual step that we have to make and would also enforce at the database layer that constraint. Same for the blobs review queue.