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
