Handle FxA login locally with a different API host

This allows logging in to zamboni running at http://localhost:2600 and using fireplace at http://localhost:8675 to handle FxA auth.

I'm not sure this is the right thing to do. It will prevent you from following the email link locally unfortunately. It might be better for zamboni to self-host the `/fxa-authorize` file for local login. I will try that tomorrow.
We do two `postMessage`s to handle app and browser use cases. One of them will always fails.