(WIP) FxA oauth support

Integration with Firefox Accounts using OAuth.
https://bugzilla.mozilla.org/show_bug.cgi?id=1041814

- [x] Implementation of the OAuth endpoints
- [x] tests
- [ ] Update the documentation to reflect these new APIs
- [ ] Update the changelog accordingly
- [ ] Test that it really works when configured properly.
The uid won't change, but the user could at some point change the email address the associate with FxA. So, if your primary key is a hash of the email, it will look like a brand new user showed up, when they would likely expect to still have their contact list (or whatever).

The `/v1/profile` endpoint returns the `email` and the `uid`, fwiw.