Add optional "Re:" prefix to all threaded mail and allow disabling mail about your own actions

Summary:

• Mail.app on Lion has cumbersome threading rules, see T782. Add an option to

stick "Re: " in front of all threaded mail so it behaves. This is horrible, but
apparently the least-horrible option.

• While I was in there, I added an option for T228.

Test Plan:

• Sent a bunch of threaded and unthreaded mail with varous "Re:" settings,

seemed to get "Re:" in the right places.

• Disabled email about my stuff, created a task with just me, got voided mail,

added a CC, got mail to just the CC.

Reviewers: btrahan, jungejason

Reviewed By: btrahan

CC: aran, mkjones

Maniphest Tasks: T228, T782

Differential Revision: https://secure.phabricator.com/D1448