hide feature flag for multiple grading periods

hide 'multiple grading periods' feature flag
for everybody except site-admins.

closes CNVS-16985
Change-Id: Ibd04232c3a9fb7131167ae5e30dcbc70d70c0168
Reviewed-on: https://gerrit.instructure.com/44618
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Spencer Olson <solson@instructure.com>
Product-Review: Spencer Olson <solson@instructure.com>