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