fix N+1 queries in AccountsController

fixes CNVS-15391

test plan: accounts api sanity check

Change-Id: I6c218ea92aec63bd3ef52072b5e9b687aeb0c886
Reviewed-on: https://gerrit.instructure.com/40840
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jon Willesen <jonw@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: Jeremy Stanley <jeremy@instructure.com>