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.
This should never get called if that's the case so I guess we can skip registering this then. We'll need to wait for `site_config` to load first though.