give more access to peer reviews

fixes CNVS-1455

test plan:

- create a graded discussion that requires peer review
- the discussion should have a peer review icon in listings
- assign peer reviews to students
- the students should have a peer review link in their
  activity stream

Change-Id: I416c876c9e21b5ce2a72c46fceb7a33bdfd3f96e
Reviewed-on: https://gerrit.instructure.com/36459
Reviewed-by: Joel Hough <joel@instructure.com>
Product-Review: Joel Hough <joel@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: David Josse <david@instructure.com>