add content_export LTI extension

test plan:
 - you should be able to enable content_export in an LTI tool
   using the External Tools API (POST or PUT) using the parameter
   "content_exports[enabled]=1"

fixes CNVS-14543

Change-Id: I777ddc867678d4eb35792938f610bdc029b9c982
Reviewed-on: https://gerrit.instructure.com/39603
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>