Fix user visible check (#21210)

Fixes #21206

If user and viewer are equal the method should return true.
Also the common organization check was wrong as `count` can never be
less then 0.

Co-authored-by: 6543 <6543@obermui.de>
Co-authored-by: Lunny Xiao <xiaolunwen@gmail.com>

models/fixtures/follow.yml

@@ -12,3 +12,8 @@
   id: 3
   user_id: 2
   follow_id: 8
+
+-
+  id: 4
+  user_id: 31
+  follow_id: 33