DA - grade calculation works on backend

fixes CNVS-13715

test plan:
  - create a student who is in one group and not another
  - create an assignment only visible to each group
  - grade the student for both assignments
  - turn DA on
    - as the student and teacher, go to the student grades page
      - both of the assignments should be visible
      - final grade should be correct (factoring in both)
        * final grade = ungraded assignments count too
    - delete one of the grades and return
      - only one of the assignments should be visible
      - final grade should be correct (using just one assignment)
  - turn DA off
    - as the student and teacher, go to the student grades page
      - both assignments are visible
      - final grade should factor in both assignments
  - with DA on and off, ensure that the following work:
    - drop rules
    - never drop rules
    - assignment stats (mean median etc)
    - what if scores

Change-Id: I727aff943b14c91089ccffa6d3b63ba026abbeec
Reviewed-on: https://gerrit.instructure.com/36762
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Mike Nomitch <mnomitch@instructure.com>