DA - submissions api includes visibility info

fixes CNVS-14080

test plan:
  - set up differentiated assignment with submission
  - as a teacher hit submissions api:show, for_students,
    and index actions
    - they can see submissions regardless of visibility
  - as a student hit submissions api:show and for_students actions
    - you should only be able to see the submission if you are in
      a secition with visibility or have a grade
    - otherwise you should get a 401 error

Change-Id: I8dcdfd5fdcc14fd6fd1c782f86599e9440348bf0
Reviewed-on: https://gerrit.instructure.com/37668
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>