crocodoc button for assignment submission page

fixes CNVS-10289
fixes CNVS-12066

test plan:
 - as a student, submit an assignment with crocodoc
 - view your submission
 - there should be a 'View Feedback' button that links you to
   the crocodoc view of the document
   - it should have a tooltip
 - as a student, submit an assignment with scribd
 - view your submission
 - there should be a 'Preview' button that has a tooltip

Change-Id: I6ead48eff4f30d95e1730d3bff905a29833b9073
Reviewed-on: https://gerrit.instructure.com/31994
Reviewed-by: Cameron Matheson <cameron@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Caleb Guanzon <cguanzon@instructure.com>
Product-Review: Cameron Sutter <csutter@instructure.com>