[WIP] Adding support for Firefox Acccounts (bug 1021296)

This is work in progress, not ready to pull in.

I'd like to get early feedback on several aspects of this feature.

1. The accounts.js file is in hearth/media/js/commonplace/, is this the right place for it?
2. I'm kicking off the accounts replacement for Persona in the callback when Persona is loaded, mainly to avoid timing issues where the Persona polyfill could overwrite the accounts polyfill.
3. The accounts polyfill is not implemented yet, working on getting a stubbed-out version to test the login flow without hitting the server, will then work on connecting to server within the flow.

That's what I can think of, but happy get get every kind of feedback.