DA - permissions

fixes CNVS-15852

test plan:
 * setup DA
 * for assignments
   - as a student, submit an assignment
   - as a teacher, remove visibility from that student
   - as a student, type in the url (/courses/:id/assignments/:id/submissions/:id)
     for your submission for that assignment
   > there should not be a link to resbumit the assignment
 * for discussions
   - as a student, using the API
   > you should not be able to reply to a discussion for which you do not have
     visibility

Change-Id: Ia4669626c745aac02ac9830865c5a5b2fd75cc37
Reviewed-on: https://gerrit.instructure.com/41947
Product-Review: Hilary Scharton <hilary@instructure.com>
Reviewed-by: Mike Nomitch <mnomitch@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>