Move assignments query in for_students to slave
Move assignments query in for_students to slave

flag=none
closes TALLY-814

Test plan:
- Have a course with some students and assignments
  - Set the course home page to use the "assignment list" view
- Smoke test loading gradebook
- As a student, smoke test loading the course home page, making sure
  scores show up for any graded submissions

Change-Id: Ibbdda033e4ed4e11b712a2361bfc29c9ae595774
Reviewed-on: https://gerrit.instructure.com/c/canvas-lms/+/233082
Tested-by: Service Cloud Jenkins <9144042a601061f88f1e1d7a1753ea3e2972119d@instructure.com>
Reviewed-by: James Williams <3a5f4ed991dbfa5c9ef76ad729d240ca12f62180@instructure.com>
Reviewed-by: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>
Reviewed-by: Jeremy Neander <20252aad5efdc666f083d82ab63ed0f9f72a043e@instructure.com>
QA-Review: Robin Kuss <0b206b54367b2de0334896ea45826f0e3194bd35@instructure.com>
Product-Review: Spencer Olson <7cc0890d6f1f70810a023e7173eca5724d6c8e49@instructure.com>

sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms,instructure/canvas-lms,sfu/canvas-lms