DA - discussions

fixes CNVS-14093

test plan:
  - turn on DA, create a graded discussion only assigned
    to one group
  - as a teacher or a student with visibility
    - you can access the dicussion through the app or the api
    - students without visibility arent included in participants
  - as a student without visibility
    - the discussion index doesn't show the discussion
    - a direct link to the discussion will send you to the
      discussion index with a flash error
    - the API doesn't show the discussion
      (show page, entries page, or replies page)
    - cannot subscribe to the topic via the api
  - with DA off, the discussion shows up normally
  - a student in both sections has visibility

Change-Id: Icefef62695f73d38787d9af08ec26c595c2e4514
Reviewed-on: https://gerrit.instructure.com/39383
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Simon Williams <simon@instructure.com>