R1:2793828795d7

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…