add foreign keys to grading_periods table

closes CNVS-16414

Change-Id: I58543fbc267350871b170b5a58644d54b9721c09
Reviewed-on: https://gerrit.instructure.com/43211
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>