use subquery instead of join for user list query

fixes CNVS-10869

avoids a large temp file being created

Change-Id: Ife71a45a96af56716c99ed02f229686a058e8d45
Reviewed-on: https://gerrit.instructure.com/29492
Reviewed-by: Rob Orton <rob@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>
QA-Review: Cody Cutrer <cody@instructure.com>