We had two additional deletion bugs lurking with review request drafts. Deleting ("discarding") the draft was failing to delete the associated ChangeDescription and DiffSet objects, causing these to be left orphaned. This was once again exposing the API bug where attempting to serialize a link to an orphaned object was crashing when it tried to get the parent object (in this case the draft that was the parent of a DiffSet that had no associated history object). Bullet-proofing the API serialization will happen separately in a later version, since that's likely to be invasive and risky, and we're close to release for RB7. This change handles the deletion issues. The one thing which is potentially still undeleted is the screenshots relation, but we no longer provide any way for users to create Screenshot objects, so that should be virtually impossible to hit. Testing Done: - Ran unit tests. - Created drafts with various types of data attached and verified that after I discarded those drafts, all related data was properly deleted.