Ask for `profile` in preverified token

https://bugzilla.mozilla.org/show_bug.cgi?id=1080868

This is the zamboni side of https://github.com/mozilla/fireplace/pull/714. Just a minor change.

Define FXA_OAUTH_URL in terms of FXA_AUTH_DOMAIN.
I was getting an error that the scope had changed when this was `'profile:email'`.