(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.
New thinking on this. We're likely going to with:

`urn:ietf:wg:oauth:2.0:fx:webchannel`

It's more consistent with what other OAuth providers are using to signal non-http redirect URIs: https://github.com/mozilla/fxa-content-server/issues/1585#issuecomment-52954338