assignments: no changing group status after students submit (frontend)

closes CNVS-11813

Test plan:
  * make an assignment
  * submit some homework
  * make sure you can't turn it into a group assignment
  * repeat the above steps for a group assignment (you shouldn't be
    able to make it an individual assignment, or change the group
    category)

Change-Id: I9fe2cb1b01972492e3707307281cee7954f8c56c
Reviewed-on: https://gerrit.instructure.com/32717
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>