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

Maniphest Tasks: T904

Differential Revision: https://secure.phabricator.com/D1762