FIX: skip category preference update if already set by group. (#20823)

`default_categories_*` site settings will update the category preferences on user creation. But it shouldn't update the user's category preference if a group's setting already updated it for that user.

app/models/user.rb

@@ -1968,7 +1968,7 @@ class User < ActiveRecord::Base
       end
     end
 
-    CategoryUser.insert_all!(values) if values.present?
+    CategoryUser.insert_all(values) if values.present?
   end
 
   def set_default_tags_preferences