R1:a4b934cad2b5

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…