Separate email formatting options into a new panel

Summary:
Ref T5861. These two options are complex, rarely useful, and not directly related to controlling what mail you receive.

Move them to a separate panel to make way for more stuff on the preferences panel. We'll probably add an "HTML" option to this new panel eventually, too.

Test Plan:

• Used both panels.
• Tested with multiplexing off.

Reviewers: btrahan, chad

Reviewed By: chad

Subscribers: epriestley

Maniphest Tasks: T5861

Differential Revision: https://secure.phabricator.com/D10236