accessibility warning for assignment group drag & drop

fixes CNVS-14750

test plan!
- on the assignment index page
- as a teacher (or other person with admin powers)
- using keyboard navigation and all the screenreaders
- tab to the assignment group drag and drop handle
- a wild accessibility warning should appear
- it should be read by screenreaders
- tab away from the drag and drop handle
- the warning should ~disappear~

Change-Id: I56d239d9dcb930b25f6ea0704cfaa7d20708aba2
Reviewed-on: https://gerrit.instructure.com/40212
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Dana Danger <dana@instructure.com>