We never really designed Review Board to delete data, and we (at least

I) also didn't fully understand how foreign keys and deletion worked

when we first designed many aspects of our database schema. The result

of this is that when a review request was deleted, we'd leave a bunch of

related things in the database, now completely orphaned. The biggest

offender here was file attachments, which not only use up space in the

database, but can also use up a lot of space on the filesystem.
This change adds a new signal handler for pre_delete on

ReviewRequest. In this handler, we explicitly delete change

descriptions, file attachments (both active and inactive), screenshots

(both active and inactive), and the diffset history.
A future change will add a management command to clean up any existing

orphaned objects.

Ran unit tests.
Individually disabled lines in the signal handler and verified that

  without this, we were leaving these objects in the DB, and with it,

  they get deleted.