discussions: separate group discussions from group assignments

fixes CNVS-4475

pre-checkout plan:
  * create a course
  * open people->groups
  * create a student group with multiple students
  * open discussions
  * create two topics
  * make one topic a group assignment

test plan:
  * open the topics you created
  * verify that a "group discussion" option replaced "group assignment"
  * verify that your group assignment is a group discussion
  * verify that your group discussion uses the same group set
  * verify that the old behavior of presenting child topics is retained
  * verify that your non-group topic is not a group discussion
  * verify that the "group discussion" flag cannot be changed for topics
      with replies

  * create a new discussion topic
  * verify that "group discussion" is available for ungraded topics
  * verify that changes to the field persist

  * create a new graded group discussion topic
  * post to one of the subtopics as a student in its group
  * open the gradebook as the teacher
  * verify that the student who posted has a submission but the other
      group members do not
  * verify that group members can be graded individually

Change-Id: Iada30628f0abdb7d7df4267d695553baf8b952d3
Reviewed-on: https://gerrit.instructure.com/33253
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Trevor deHaan <tdehaan@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
Product-Review: Braden Anderson <banderson@instructure.com>