give content_exports polymorphic context

test plan:
* content export regressions

closes #CNVS-13468

Change-Id: I44dcf077967d4273c0ae9056074b03853d66a5dc
Reviewed-on: https://gerrit.instructure.com/35745
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>