fix assignments_needing_x sharding issues

course_ids were being looked up on the currently active shard and then used on
the user shard

fixes CNVS-14449

test plan:
- create a teacher and two shards, each with a course
- enroll the teacher in both courses (so they are now a cross-shard user)
- in each course, create a student and have them submit an assignment
- load the dashboard on each shard
- both shards To Do lists should be the same -- they should both list
  2 assignments (one from each shard/course) as needing grading

Change-Id: Id34541f7d7144fa0ffb8b221a95aa9369118f50a
Reviewed-on: https://gerrit.instructure.com/38372
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: August Thornton <august@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>