create media objects on the account's shard

fixes CNVS-9711

Change-Id: If6f49101350e8adca55254906f45f5c3381af74b
Reviewed-on: https://gerrit.instructure.com/34509
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jacob Fugal <jacob@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>