TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
Yes, I think "empty-course" should be removed. The test case that I am expecting is some combination of user and courses where some course but not every course only contains updates from a cohort other than that of the user, with an assertion that such courses do not appear in the final digest for that user.