(WIP) FxA oauth support

https://bugzilla.mozilla.org/show_bug.cgi?id=1041814
The client code is expecting to get the OAuth token so it can talk to the profile server directly: https://bugzilla.mozilla.org/show_bug.cgi?id=1047146

I'm not sure what endpoint returns that to the client, but it could be `POST /fxa-oauth/oauth`.