fix public syllabus access

test plan:
 1. have a published course containing at least one published
    assignment
 2. check the "public syllabus" box in course settings
 3. the syllabus page should be visible as a non-logged-in user

fixes CNVS-14144

Change-Id: I5f5f357227076e657a9626cc3748ebcf0be7365e
Reviewed-on: https://gerrit.instructure.com/37528
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>