Drive Differential review request e-mail message by field specification

Summary:
This also changes some stuff:

• Reviewers used to be at top, now they are under comments.
• Primary reviewer is now rendered as first.

Test Plan: Added renderValueForMail() to a custom field, created diff, commented on it and verified e-mails.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2664