"display_all" is too coarse, and we should be letting event handlers
make the appropriate decision on what to display and when. This
duplicates some code, but it's now very clear in the event handlers
what's getting shown.
Throw a 404 if we try to view the user profile for a missing user.
The only feature change in this should be that we now display the
name, full name and website for a user to any other registered user,
which makes sense since these are typically public fields.
Don't show any of the edit buttons unless identity::is_writable()
