prevent N+1 query on sub_accounts#show

fixes CNVS-14835

Change-Id: Iaabf7d1994f355abf3ea000bdbcef9e208f54712
Reviewed-on: https://gerrit.instructure.com/39448
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: James Williams  <jamesw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>