This change adds a new management command that will clean up various

pieces of data in the database and filesystem that could have been left

behind if administrators used the "Delete Permanently" action on a

review request or deleted review requests via the admin UI.
It will query each of the affected items to get a total count, and then

perform the actual deletion in batches. This allows us to show nice

progress bars, as well as make the command interruptible.

Populated my database with a bunch of data including screenshots,

file attachments, diffs, and change descriptions. Temporarily disabled

our new signal handler and deleted the review requests.

Ran with --show-counts-only and saw the expected counts for each type

  of object.
Ran normally and watched it delete all the objects.
Ran again with --show-counts-only and saw that it no longer reported

  any orphaned data.   
Set the batch size low and verified that the progress bars worked

  correctly.
Built and looked at the docs.