keep crocodoc off canvadocs strand

refs CNVS-16092

Test plan:
  submission attachments should still be previewable in crocodoc

Change-Id: Ieee05820ecd19c70142876ec2ed987cac133ccc0
Reviewed-on: https://gerrit.instructure.com/42868
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Spencer Olson <solson@instructure.com>
Reviewed-by: Josh Simpson <jsimpson@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>