DA - gray out gb2 cells

fixes CNVS-9767

test plan:
 * with the differentiated assignments feature flag on
 * as a teacher
 * with assignments that have due dates for only some of the sections
 - view gb2
 > any cells whose student is not in a section that has a due date
   for that assignment should be grayed out
 * with the differentiated assignments feature flag is off
 - open gb2 and make sure things don't explode

Change-Id: I0906dc1dc53594e4c7cbc99a88acef2b50d0e325
Reviewed-on: https://gerrit.instructure.com/37229
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>