DA - speed grader shows right students and sections

fixes CNVS-13810

test plan:
  - with DA on create multiple assignments visible to
    select sections
  - go to speed grader for said assignments
    - only students with visibility should show up
    - only sections with visibility should show up
  - turn DA off and everything still works
  - note: group assingments not done yet

Change-Id: Ia065a82a24db40cc6ba46586114167e2ea8e75c1
Reviewed-on: https://gerrit.instructure.com/36981
Reviewed-by: Cameron Sutter <csutter@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>