DA - assignments api index/show

fixes CNVS-14074

test plan:
 * setup DA
 * DA feature flag on
 - use the API for the following requests
   - index
     > should only show assignments that the student can see
   - show
     > should return as assignment that the student can see
     > should return an error for assignments that the student
       cannot see

Change-Id: I921ffbab639d96c3bfbf16d0b1df9ef79515f44e
Reviewed-on: https://gerrit.instructure.com/39520
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Liz Abinante <labinante@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>