DA - send flag from assignment edit page

fixes CNVS-9765

test plan:
 - turn on the Differentiated Assignments feature flag
 * from the edit page
  - add a due date for some of the sections, but not all
  - save (update assignment)
  - the only_visible_to_overrides flag on the assignment
    should be true
    (you'll need to check the assignment with the API)
  - edit the assignment again and there should not be an
    empty due date row for 'Everyone else'
  - add a due date for all the sections
  - update the assignment
  - the only_visible_to_overrides flag should be false
 * on the show page
  - edit the assignment so that there are only due dates
    for some sections but not all
  - there should not be a row in the due date area
    for 'Everyone else'

Change-Id: I1bce92c335c1af2c9ead6d963174c817d4b10cdd
Reviewed-on: https://gerrit.instructure.com/33706
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>