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.
- Switched preference to default (saw "HTML" in UI).
- Sent myself some mail.
- Got HTML mail.
Reviewed By: chad
Maniphest Tasks: T10694
Differential Revision: https://secure.phabricator.com/D15885