create differentiated assignments feature flag

fixes CNVS-9763

test plan:
 - the differentiated assignments feature flag
   should be available on the account and course levels
 - you should be able to allow or turn off on the account level
 - you should be able to turn on or off on the course level
   if it's allowed on the account level

Change-Id: Iabb96881fb7c039198ee13af579873baeab9ce4b
Reviewed-on: https://gerrit.instructure.com/29283
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>