Clean up Differential draft mail behaviors
Summary:
Ref T2543. Fixes two relatively minor things:
- When builds finish in Harbormaster, send mail "From" the author.
- Set the `firstBroadcast` flag so that initial mail picks up earlier history (notably, the "reviewers" line).
For now, I'm not setting `firstBroadcast` on explicit "Request Review" (but maybe we should), and not trying to deal with weird cases where you leave a bunch of comments on a draft. Those might be fine as-is or may get tweaked later.
Test Plan: Created a revision with Harbormaster builds, ran builds, saw initial email come "From" the right user with more metadata.
Reviewers: amckinley
Reviewed By: amckinley
Maniphest Tasks: T2543
Differential Revision: https://secure.phabricator.com/D18748