retain assignment muted status on course copy/import/export

test plan:
* create a muted assignment
* copy the course, or use import/export
* the migrated assignment should also be muted

refs #CNVS-12483

Change-Id: I124141ae088cb70a4d93f4270c5ad103c4b24a3a
Reviewed-on: https://gerrit.instructure.com/33415
Reviewed-by: Bracken Mosbacker <bracken@instructure.com>
Product-Review: Bracken Mosbacker <bracken@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Nathan Rogowski <nathan@instructure.com>