MGP - no permissions for grading period without feature flag on

fixes CNVS-15968

test plan:
 * using the API
 * with the Multiple Grading Periods flag off
 - try to create/read/update/delete a grading period
 > it should return a 'not found' error

Change-Id: I92ed920fde7f03c4ac9a051d4a7212bf43a53fa3
Reviewed-on: https://gerrit.instructure.com/44552
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>