retain unpublished state for discussion topics on copy

test plan:
* create an unpublished discussion topic and a
 published one
* copy the course (or export/import it)
* should retain their respective states

closes #CNVS-14201

Change-Id: If7702b67701b50ce4e2a027d1d4b5ab441d86003
Reviewed-on: https://gerrit.instructure.com/37900
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>