HTML emails

Summary:
Added support for side-by-side HTML and plaintext email building.

We can control if the HTML stuff is sent by by a new config, metamta.html-emails

Test Plan:
Been running this in our deployment for a few months now.

====Well behaved clients====
 - Gmail
 - Mail.app

====Bad clients====

- [[ http://airmailapp.com/ | Airmail ]]. They confuse Gmail too, though.

====Need testing====
 - Outlook (Windows + Mac)

Reviewers: chad, #blessed_reviewers, epriestley

Reviewed By: #blessed_reviewers, epriestley

Subscribers: webframp, taoqiping, chad, epriestley, Korvin

Maniphest Tasks: T992

Differential Revision: https://secure.phabricator.com/D9375