don't let crocodoc blob/hmac be used for canvadoc

Test plan:
  make sure canvadocs/crocodoc still work in speedgrader

Change-Id: Ie5a7b8759206b0293757a2c2d820f0e7f5af164d
Reviewed-on: https://gerrit.instructure.com/36201
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>