disable "must contribute to the page" requirement for assignments

test plan:
 with draft state off and on, confirm that the "must contribute
 to the page" requirement is grayed out for assignments
 in the Edit Module dialog

fixes CNVS-12624

Change-Id: I46d0c8264ef43ac81ed3a59203956f8c04989d2c
Reviewed-on: https://gerrit.instructure.com/33792
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>