DA - assignment indices

fixes CNVS-13708

test plan:
  - create a student who is in one section and not another
  - create an assignment only visible to each section
    (1 section => 1 assignment)
  - grade the student for both assignments
  - turn DA on
    - as the student, go to the assignment index page
      - both assignments should be visible initially
      - delete the grades for both assignments and return
        - only the overridden assignment should be visible
      - only one of the assignments should be visible
    - as the student, go to the grades page
      - only the overridden assignment should be visible
      - regrade the assignment whose grade you had deleted
        - both assignments should be visible now
      - (fyi: final grade wont be correct)
  - turn DA off
    - as the student, go to the assignment index page
      - both assignments are visible
    - as the student, go to the grades page
      - both assignments are visible
      - final grade should factor in both assignments

Change-Id: I32e3fea4d6fd3db9a512e0d170779b48418f90cf
Reviewed-on: https://gerrit.instructure.com/36728
Reviewed-by: Liz Abinante <labinante@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>