add index on delayed_messages (notification_policy_id) to support a foreign key

Change-Id: Ia46f2cdafbb8efbf7e3ceafb5b1a4c2b81cf27c1
Reviewed-on: https://gerrit.instructure.com/29928
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>