look up user notification policies on the correct shard

fixes CNVS-15829

test plan:
- in a course that contains a user from a different shard
- change a due date on an old assignment (at least 3 hours old)
- all appropriate users should get a dashboard notification

Change-Id: I36c92e91be495dfb5ccaa6ce592af97780f6a678
Reviewed-on: https://gerrit.instructure.com/41830
Reviewed-by: Cody Cutrer <cody@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>