add group and user file index endpoints

test plan:
 - consult the "List files" API endpoint documentation
   you should see course, group, user, and folder versions
 - exercise the group and user calls

fixes CNVS-13827

Change-Id: Ic3b0b374254a6a0ef0c986b43b9b29c08b98f202
Reviewed-on: https://gerrit.instructure.com/37032
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Ryan Shaw <ryan@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>