DA - gb2 visibility data

fixes CNVS-9872
refs CNVS-14074

test plan:
 * turn on the differentiated assignments feature flag
 * edit an assignment so that not all sections can see it
 * using assignment groups API
 - in the request make sure to include "assignments" and "assignment_visibility"
 > there should be an assignment_visibility object within each assignment object
 > the assignment_visibility object should be an array of unique ids
 > the ids should only be students in sections that can see the assignment
   or students whose submissions were graded
 * using the assignments API
 - in the request make sure to include "assignment_visibility"
 > there should be an assignment_visibility object
 > it should be an array of unique ids
 > the ids should only be students in sections that can see the assignment
   or students whose submissions were graded

Change-Id: I68e832c3dc1859e398f656fa77804b2733f0dcd3
Reviewed-on: https://gerrit.instructure.com/36811
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>