add limited view functionality to accounts api for teachers

test plan:
* as a teacher, ta or designer,
 the use the api to view basic information about
 the accounts that own your courses (using the regular
 "show" action and the "course_accounts" endpoint

close #CNVS-15847

Change-Id: I7109e9af43278ab0f2635fb94dde5795873ae0cc
Reviewed-on: https://gerrit.instructure.com/41909
Product-Review: Hilary Scharton <hilary@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
Tested-by: Jenkins <jenkins@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>