TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
The docstring for `sorted` says: `Return a new sorted list from the items in iterable.`  And for a dict, only its keys are iterated, by default.

But I can see that the code would be clearer if it explicitly says `.keys()`.  Since this is not a performance-sensitive area of the code, I agree we can add this extra explicit step.