make drop down for admins limit to active accounts

fixes CNVS-13839

test plan
 - create an account
 - create an admin
 - delete the account
 - the admin role should not show in the drop down

Change-Id: I12c3f6cc615deb0646d3b4f2af60bb191f070edc
Reviewed-on: https://gerrit.instructure.com/37076
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Cody Cutrer <cody@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Cody Cutrer <cody@instructure.com>