Developers banner and popup (bug 1052880)

https://bugzilla.mozilla.org/show_bug.cgi?id=1052880

FxA migration banners on devhub.


![fx-migration-desktop mov](https://cloud.githubusercontent.com/assets/211578/4412923/5044356c-44fe-11e4-8378-f7a5c9bdf695.gif)

I added a `console.error("Error parsing settings as JSON.");` on the next line.