fix attachment previews in submissions api

fixes CNVS-15471

Test plan:
  * make an assignment
  * upload some canvadocable (or crocodocable) file submissions
  * make sure the submission api includes the preview url for the
    documents

Change-Id: I8223d9e0cbd51b575b27cfd9a46a6122255e88c9
Reviewed-on: https://gerrit.instructure.com/41010
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>