fix NotificationPolicy.setup_for to be shard safe

Change-Id: I45b9f0c7fcf2bd75ee9a45155d2fb7eb9332a049
Reviewed-on: https://gerrit.instructure.com/31154
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Anthus Williams <awilliams@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>