canvadocs

This commit adds support for document previews from a Box View
compatible API.

closes CNVS-12416

Test plan:
* NOTE: check everywhere for previews (speedgrader, files list,
  individual files page, etc?)
* Enable Scribd and Crocodoc
  * attachments should be previewable in scribd or crocodoc (submission
    attachments should go to Crocodoc)
* Enable Canvadocs
  * Files that previously previewed in scribd should now go to
    Canvadocs.  Nothing should ever go to scribd.
  * New submission attachments should continue to use Crocodoc (for
    supported file types)
  * Other attachments should preview in Canvadocs

Change-Id: I173e4fabc0ae677cdddd4bd065777a90360c1f34
Reviewed-on: https://gerrit.instructure.com/34535
Reviewed-by: Simon Williams <simon@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Matt Fairbourn <mfairbourn@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>