make canvadocs work in public courses

fixes CNVS-13805

Test plan:
  * make a public course
  * upload a canvadocable file
  * visit the course in a new browser (without logging in)
  * you should be able to preview the file

Change-Id: I4ef0b0ba656f2aad734736178b09ad9a729ae3c9
Reviewed-on: https://gerrit.instructure.com/37245
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>