This changes the two diff upload paths (JSON and view) to
create a new draft and attach the diff to that instead of
writing it to the DiffSetHistory and sending an e-mail
immediately.  This has been requested by a few folks since
they like to look over the diff to make sure it's correct
(viz. changes, trailing whitespace, etc.) before actually
making it public.

As part of this, I'm introducing some more fine-grained
change tracking when we save a draft.

Uploaded diffs, checked that various objects I expected were
in the database.  Added print statements to see whether email
code paths were correct.

This probably needs a lot more; I'll do some on reviewboard-test
to make sure post-review and actual e-mails work.