DA - needs grading

fixes CNVS-15482

test plan:
 * with DA on
 - create an assignment and submit it as several students in different sections
 - as a teacher again, change the assignment to only be visible to one section
   (one that has student submissions)
 > on your dashboard, only the students' submissions from that section
   should show in your 'needs grading' right-side bar
 > on the course dashboard the same should be true
 - as a section limited teacher who is limited to the section with visibility,
   your dashboard and course dashboard should only show
   submissions from your section as needing grading

 * with DA off
 - repeat the steps above and all submissions should be in the needs grading
   count in all places

Change-Id: I6c2fad7d7dea42c2e7edc968e8db113bdcab3a3c
Reviewed-on: https://gerrit.instructure.com/41604
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>