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

• 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