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.
This just makes it a little easier to change to the latest server in development.