Reduce database queries when returning the User list resource.
The User API resource checked profile.is_private on each User being

serialized in a list, which is expensive. Unfortunately, neither

select_related() nor prefetch_related() appear to properly cache reverse

ForeignKey relations, so they don't help us with this.
The best solution, it seems, is to tack on an extra query for our users,

storing the result of Profile.is_private on the User. The

is_profile_visible function checks for this, and attempts to do the

right thing by making use of that value, if it's available.

Viewed the query count and list of queries before and after this change.

Before the change, there was an extra query per listed user. After,

there was a flat amount of 4 (which includes session and auth queries).