order exports in /courses/X/content_exports

fixes CNVS-14765

test plan:
 - have a course with multiple content exports
 - /courses/X/content_exports should display them in order
   by date, descending

Change-Id: I76356ab83b9631dd192dea18c331b425505a92d2
Reviewed-on: https://gerrit.instructure.com/39252
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>