Only show FxA banner for migratable users (bug 1070008)

https://bugzilla.mozilla.org/show_bug.cgi?id=1070008

Only show the FxA banner if the user hasn't signed in through FxA and the user has logged in at some point.
If finds the banner and closes it. I guess it should use `document.getElementById` since the `window['id']` thing is a little obscure.