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.
This was supposed to be a POST endpoint because it mutates state on the server. Has this changed? Also, I implemented this with /fxa-oauth/params to match the documentation but this is now /fxa-oauth/parameters. Can you please keep me in the loop for changes like these? The Firefox code in the tree doesn't work because of these two issues.