DA - assignment and submission access for students and observers

fixes CNVS-14071

test plan:
  - set up differentiated assignments
    - as a student go directly to an assignment url
      (eg - courses/1/assignments/2)
      - students without visibility should get a flash
        message and get redirected to the assignment index
      - students with visibility should see the normal page
    - as a student go directly to a submission url
      (eg - courses/1/assignments/2/submissions/3)
      - students without visibility should get a flash
        message and get redirected to the assignment index
      - students with visibility should see the normal page
    - as a teacher and TA these pages should work fine
    - observers should have the same permissions as the
      students they are observing (try hard to break)

Change-Id: I063fbd5ab3664d69754f01f5bdc740fb79048804
Reviewed-on: https://gerrit.instructure.com/37704
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>