DA - avoid seq scans in scopes

test plan:
  - with DA on and OFF
  - assignments & discussions & quizzes & modules indices load
    and filter properly for students/teachers
  - assignments needing submitting/grading load and
    filter properly
  - grade summary page for students & teachers loads
    and filters assignments properly

Change-Id: Ibff8ff0357360a7e09d500c2475885f3dfb7f495
Reviewed-on: https://gerrit.instructure.com/44215
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>