never drop accessibility

fixes CNVS-12823

test plan:
 * on the assignment index page (with DS)
 - open the assignment group edit dialog
 > the neverdrop rules should be accessible
 > the dropdowns to select which assignments should never be dropped
   should also be accessible

Change-Id: Icefd8a0ab5184886232765136fa40da3bab0179d
Reviewed-on: https://gerrit.instructure.com/34857
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>