TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
I removed "empty-course".
I think the test you are asking for is already there.  In the `test_cohort_filtering` method, the payload contains digest information for cohorts that the user does not belong to.  The set equality assertions then make sure the filtering has worked correctly.
https://github.com/edx/notifier/pull/23/files#diff-3cd71a41627aa54ad3a3bf19b9a072f1R277