Refactor setting e-mail subjects

Summary:
It seems that Outlook and Mail.app mostly ignores the threading headers and thread primarily by subject.
They are also very picky about the Re: part in the header.
I guess that's because users of these clients often hit Reply when they want to create a new message to the sender of an e-mail.

We need both of these applications to work with the same setting because we don't use multiplexing to prevent sending multiple e-mails to people in lists.
I also believe that the default behavior should just work in most setups.

I've tried several different combinations of putting "Re:" and none of them seems to always work in both clients.

This diff at least adds more abstraction to the code which should prevent copy/paste errors (two fixed by this diff!).

Test Plan: Sent several e-mails with varying subject, verified that they look as before in Outlook and Mail.app.

Reviewers: epriestley, nh

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2709