DA - eportfolio discussion link

fixes CNVS-16170

when a user cant see a DA'ed assignment there
is no longer a link to access it

test plan:
  - make a DA'ed discussion
  - make some comments on as a student
  - take away visibility to this student
  - add it to your eportfolio as the student
  - there should not be a link to go to the discussion
    when viewing as the student
  - as the teacher or a student with visibility, there
    should be a link to go to the discussion

Change-Id: Id72c39985f74b2f52b27edf70923bcb9d75f7ab2
Reviewed-on: https://gerrit.instructure.com/42973
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cameron Sutter <csutter@instructure.com>
QA-Review: Sean Lewis <slewis@instructure.com>
Product-Review: Hilary Scharton <hilary@instructure.com>