add verifiers to attachments in public syllabus content

test plan:
* create a course with a public syllabus
* add embedded images to the syllabus
* view the syllabus as a public user  (not logged in)
* should still see the images

closes #CNVS-17174

Change-Id: I03ae8d2efddb7b50b7503f50e54937cf00aefe4b
Reviewed-on: https://gerrit.instructure.com/45336
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Jahnavi Yetukuri <jyetukuri@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>