Send HTML email by default

Summary: Ref T10694. Switch default mode to HTML since it has a number of significant advantages and we haven't seen reports of significant problems.

Test Plan:

• Switched preference to default (saw "HTML" in UI).
• Sent myself some mail.
• Got HTML mail.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T10694

Differential Revision: https://secure.phabricator.com/D15885