don't add verifiers to locked file links

test plan:
 - have a course with a public syllabus
 - link to two files in the syllabus body:
   - a locked file
   - an unlocked file
 - access the course syllabus without logging in
   - the unlocked file link should work
   - the locked file link should not

fixes CNVS-11569

Change-Id: I75d25a46c7e4fac43c7f187a4aba166be85d1010
Reviewed-on: https://gerrit.instructure.com/31270
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>