R1:ad36865e504c

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…