enforce no changing group_category on assignments after submissions

refs CNVS-11813

Test plan:
  * create an assignment
  * assign a group category to it (this should work)
  * remove the group category (this should work)
  * submit some homework
  * use the API to try and change the group category.  it won't let you

Change-Id: I09a56e281956d30e9213d6105bd9c75ede97da1c
Reviewed-on: https://gerrit.instructure.com/32719
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>