TNL-301 Filter posts by group_id.

https://openedx.atlassian.net/browse/TNL-301

@gwprice @jimabramson Please review.
https://github.com/edx/cs_comments_service/blob/master/lib/helpers.rb#L312

cs expects and stores a number; i don't see anything between the output of the cs service call and here that stringifies.