Send custom headers for author, reviewer and cc in Differential e-mails

Summary:
I want to flag messages which require an immediate action from me in e-mail
client.
It is currently not possible because Author and Reviewers fields are both in
To:.
So the filtering rule cannot recognize if I am the person who should take the
action.

This diff adds these headers:

• X-Differential-Author
• X-Differential-Reviewers
• X-Differential-CCs

Test Plan:
Send comment to the diff.
Verify X-Differential-* headers.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, epriestley

Maniphest Tasks: T808

Differential Revision: https://secure.phabricator.com/D1724