Replace AnonymousUser with User.new.

* Replace AnonymousUser null object with a readonly, unpersisted User object.

* Default always_resize_images to true (previously it was true for
  anonymous users, but false for new members).

* Default comment_threshold to -1 for anonymous users (previously it was
  0 for anonymous but -1 for new members).

app/logical/session_loader.rb

@@ -11,7 +11,7 @@ class SessionLoader
   end
 
   def load
-    CurrentUser.user = AnonymousUser.new
+    CurrentUser.user = User.anonymous
     CurrentUser.ip_addr = request.remote_ip
 
     if Rails.env.test? && Thread.current[:test_user_id]