attempting to fix the issue with override_final grades not showing up in...

Ok @bufke , now that I've successfully got this into a pull request, can you take  look at this changes and make sure everything looks honkey-dorey?

Specifically, what was happening was that the API was filtering out grade objects that had null marking periods, which included grades that were "override_final" grades (by design, these have no associated marking periods). I was looking around and found away to use django's Q() to create a condition where these override_final grades don't get filtered out.