TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
That's true if the API is now a new endpoint.
But I was working under the assumption that we needed to support backward compatibility with the old User endpoint (where there may not be any `course_info`).
I can update the code as you suggest.