This adds a handler for performing extra cleanup when review request drafts

get deleted. Right now the only thing that we do in the handler is delete all

draft file attachments that are being discarded alongside the draft. That

fixes a bug where discarding a review request draft that contained an

update to a file attachment would erase the entire file attachment from the

review request, rather than reverting the file attachment to its previously

published version. It also makes sense to not keep the draft file attachment

around, since it will never be published/accessed again after being discarded.

Ran unit tests
Made sure that the bug was fixed.