DA - group assignment alert

fixes CNVS-14929

test plan:
  - with DA on
    - make a group assignment
    - you should get a warning in the group assignment box
    - publish the assignment and turn in homework from a student
    - go back to the edit page
    - you should see a different warning in addition to the
      original warning
    - repeat steps for a graded group discussion
  - with DA off
    - there should not be an error related to DA

Change-Id: Id2ca3c12a50df009cfd9273d8a0f0fd778f81646
Reviewed-on: https://gerrit.instructure.com/40870
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>