Only register one FxA message handler (bug 1080347)

https://bugzilla.mozilla.org/show_bug.cgi?id=1080347

Register the FxA message handler when `login.js` is loaded instead of when the "Sign In" button is clicked so we only ever register one handler.
Because it will register a callback each time `startLogin` is called and we'll send the token to zamboni as many times as you've clicked "Sign In" this session.