Bug 938896 - Find My Device

Most of the sign-up/login code is not really final, as we expect to move from Persona to Firefox Accounts in the near future. My rough goal when we finally switch is to interact with Firefox Accounts entirely from the System app, and there will be no need for the iframe trick in apps/settings/js/findmydevice.js

It probably makes sense to land this disabled somehow. I'm not exactly sure how this works in Gaia, so I'd love to hear suggestions for how to make this "preffed off".