Filter commentable queries by course_id

Bug: TNL-207
Merge pull request #117 from edx/gprice/commentable-course-id-filter

Filter commentable queries by course_id
Add group_id to payload from notifications.

TNL-299
Merge pull request #118 from edx/christina/notifications_group_id

Add group_id to payload from notifications.
Add the ability to query multiple group ids