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.
It's just easier than deciding if it is already shown and changing it's text when the user switches from logged in to logged out and vice versa.