Bug 1041814 — 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
- [x] Update the changelog accordingly
- [ ] Test that it really works when configured properly.
:+1: yeah we need the `content_uri` to verify the origin in Firefox.