Don't create stream items for unpublished discussions

Fixes CNVS-13979

Test plan:
  - As a teacher with access to a given course, create, but do not publish,
    a new discussion topic in said course
  - As a student enrolled in said course, hit
    /api/v1/users/self/activity_stream and verify that the discussion
    does not appear in the list
  - As the teacher, publish the discussion
  - As the student, hit /api/v1/users/self/activity_stream and verify that
    the discussion now appears in the list
  - As the teacher, unpublish the discussion
  - As the student, hit /api/v1/users/self/activity_stream and verify that
    the discussion no longer appears in the list

Change-Id: I8f6a75d9b4946f92adf7a5e413ecca0c6b00d65b
Reviewed-on: https://gerrit.instructure.com/40603
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Janelle Seegmiller <jseegmiller@instructure.com>
Reviewed-by: Matthew Wheeler <mwheeler@instructure.com>
QA-Review: Steven Shepherd <sshepherd@instructure.com>