let account users with :read_roster rights view enrollments

test plan:
* create an account role
* grant the account role the ability to "See the list
 of users"
* as a user with that role, should be able to view the
course enrollments on a user's details page

closes #CNVS-14169

Change-Id: I7e5256af1e52118f14799c6a76603de61eb14f25
Reviewed-on: https://gerrit.instructure.com/39203
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jeremy Stanley <jeremy@instructure.com>
QA-Review: Clare Strong <clare@instructure.com>
Product-Review: James Williams  <jamesw@instructure.com>