R1:5590515007f0

Make Diffusion Herald emails thread with Audit emails

Summary:
- Users may elect to receive an initial notification about a commit; allow it to be replied to in order to interact with the object.
- Share thread headers between emails.
- Add the "REPLY HANDLER ACTIONS" section to both emails.

Test Plan:
- Used "reparse.php --herald" to trigger herald emails, verified reply-to and email body.
- Made audit comments, verified body.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran, epriestley…