Two modes of visibility checking are supported, depending on the value
of the 'messagingallusers' site setting (site-wide messaging):
- If site-wide messaging is enabled, a user may only be returned in
search results if the searching user can view their profile somewhere
(either the site profile or any course profile).
- If site-wide messaging is disabled, a user may only be returned in
search results if the searching user shares a course with them and can
view their course profile in the shared course.