'require_usage_rights' feature flag

test plan:
 - as site admin, set feature to 'allowed' in a root account
 - as a teacher in a course under this account, see that feature
   becomes available to be allowed in course settings

closes CNVS-15864

Change-Id: I569e6febf44bcf75a46bcbbbc25d15a99571cc46
Reviewed-on: https://gerrit.instructure.com/44567
Reviewed-by: Clay Diffrient <cdiffrient@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: Cosme Salazar <cosme@instructure.com>