canvadocs: rich-text editor support

fixes CNVS-12976

Test plan:
  * previews in user content should now open in canvadocs instead of
    scribd.
  * disable scribd.
  * add some canvadocable files to a wiki page (or wherever).  those
    files should preview in canvadocs.

Change-Id: I320984ad4d860af266cf83b3edaad44e76c120fc
Reviewed-on: https://gerrit.instructure.com/34686
Reviewed-by: Ryan Shaw <ryan@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>