Prepare revision mail for the "Draft" status

Summary:
Ref T2543. Currently, we always do some special things when a revision is created, mostly adding more stuff to the mail.

With drafts, we want to suppress initial mail and send this big, rich mail only when the revision actually moves out of "draft".

Prepare the code for this, with the actual methods hard-coded to the current behavior. This will probably take some tweaking but I think I got most of it.

Test Plan: Banged around in Differential so it sent some mail, saw normal mail without anything new.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T2543

Differential Revision: https://secure.phabricator.com/D18627