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. Testing Done: - 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.