data model and API for file usage rights

test plan:
 1. consult the files API documentation and note the additions:
   A. usage_rights include parameter on 'List files'
   B. usage_rights include parameter on 'Get file'
   C. 'List licenses' endpoint
   D. 'Set usage rights' endpoint (PUT)
   E. 'Remove usage rights' endpoint (DELETE)
 2. exercise these new API features

closes CNVS-16826
closes CNVS-16841

Change-Id: I6c01128423adc24a5b70f71bb109d5a005379c37
Reviewed-on: https://gerrit.instructure.com/44403
Reviewed-by: James Williams  <jamesw@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Anna Koalenz <akoalenz@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>