use linked assignment for discussion regrouping check

fixes CNVS-14003

test plan
- create a graded group discussion
- make the topic like a pre group assignment/graded group
 discussion separation commit by doing the following in a console
 - set the linked assignment's group_category_id to the topic's
  group_category_id
 - set the discussion's group_category_id to nil
- edit the discussion from the web ui
- ensure that the discussion can be edited

Change-Id: I7fc385e0ce2236f5986cf601b46a057708c81079
Reviewed-on: https://gerrit.instructure.com/37469
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: David Josse <david@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>