Pass group_id when getting user thread and comment counts.

TNL-543

Related cs_comments PR is https://github.com/edx/cs_comments_service/pull/122

@jimabramson @dan-f Please review.

Can use my sandbox to test--
http://cahrens.m.sandbox.edx.org/courses/christina/foo/bar/discussion/forum

This course has the following users and posts:
1) staff@example.com has moderator access. staff has started 3 discussions-- one to general (not cohorted), one to Verified Group, one to Honor Group. staff has also commented on two posts-- one written by honor to everyone, and one written by honor just to Honor Group cohort.

2) honor@example.com is in Honor Group. honor has created 2 posts, 1 to everyone and 1 to Honor Group. honor has also commented on both of those posts.

3) verified@example is in Verified Group. verified has created 2 posts, 1 to everyone and 1 to Verified Group.
Sure, I'll change the test so it expects exactly one request to have a course_id specified.