Bug 847168 - [User Story] Email Signature

It looks like you started to centralize the handling of the signature form controls here, but did not complete it. I would like to see that completed, since it means we only need the JS logic for that wiring here instead of duplicating it in setaup_account_prefs.js and settings_account.

We could even look at centralizing the injection of the HTML too if you wanted. Although, I am fine if we pass on that for now and wait until we have our custom element story sorted out, since the other centralized form controls in here still duplicate their HTML.