fix canvadoc locked-files issue

fixes CNVS-14192

Test plan:
  * Make a course with a canvadocable file
  * Add the file to a wiki page
  * Lock the file
  * Students viewing the wiki page should not be able to preview the file

Change-Id: Ic706085492b07db24dfd48469bcb3e76e1e7e609
Reviewed-on: https://gerrit.instructure.com/37602
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Hannah Bottalla <hannah@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>