DA - let students see old submissions

CNVS-15685

test plan:
  - DA ON:
    - make a differentiated assignment and assign to student
    - as student, turn in a submission
    - as teacher, take a way visibility
    - as student, use a direct link to get to yoru assignment
      - you should be able to see it and get a flash notification
  - DA OFF:
    - you can also see the submission but get no notification

Change-Id: I01cebc5d2a12ed769496d6f61a2a81adedd74241
Reviewed-on: https://gerrit.instructure.com/41443
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>