When I added change descriptions and diffsets to the draft deletion

handler, I wasn't thinking about how we delete the draft after

publishing it. In that case, we've assigned the change description to

the review request, and assigned the diffset to the history, but we

haven't cleared them from the draft object. We were therefore deleting

the data right after we published it.
This change fixes that up to properly check that those objects are no

longer alive.
Unit tests have been added for this case with both of those as well as

file attachments, which didn't have this same problem because of the way

we calculate the file attachment state.

Ran unit tests.
Published a new review request that included a diff and saw that my

  diff wasn't immediately deleted.