Bug 847168 - [User Story] Email Signature

Let's move this into the back-end and document why we're doing it with a comment in there.  We should also update the modifyIdentity tests to verify that the change happens instantaneously.  In general I'm a bit conflicted about lying to the user about state (if we're broken, it's better to actually be broken), but this is arguably infallible as far as the back-end can tell and there's basically a compulsory glitch otherwise.