DA - correct grading in gb

fixes CNVS-16491

test plan:
 * setup DA
 - navigate to gradebook
 > assignments that are not visible to a student should not affect
   her grades
 - turn on "Treat ungraded as 0"
 > assignments that are not visible to a student should still not
   affect her grades

Change-Id: I1ad58b2984a4306cff89038a3d46d7f3c99c8fce
Reviewed-on: https://gerrit.instructure.com/43528
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>