include document preview urls in submissions api

closes CNVS-13322

Test plan:
  * consume endpoints on the submissions api
  * attachments should now include a "preview_url" (it will display the
    document)

  * document previews should still work everywhere in canvas
    (speedgrader, files, tinymce)

Change-Id: Ic11725832fa55e0deedf3bbce4a8b6a84f4ea29a
Reviewed-on: https://gerrit.instructure.com/36423
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Simon Williams <simon@instructure.com>
QA-Review: Amber Taniuchi <amber@instructure.com>
Product-Review: Cameron Matheson <cameron@instructure.com>