discussions: prevent creation of new group graded discussions

test plan:
  * create a graded discussion
  * using the Assignments API, try to set group_category_id
      on the discussion's linked assignment
  * verify that you get an error message
  * using the Discussion Topics API, set a group_category_id
      on the discussion
  * verify that the discussion becomes a group discussion

Change-Id: Ida1bae1f370a5d5f13a5002b9cdeb40c24dba0b2
Reviewed-on: https://gerrit.instructure.com/35084
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Braden Anderson <braden@instructure.com>