DEV: Add true_fields method for CustomFields (#24876)

This is useful for plugins that might otherwise rely on the
CUSTOM_FIELD_TRUE constant.

app/models/category_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class CategoryCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :category
 end
 

app/models/concerns/custom_field.rb

@@ -0,0 +1,11 @@
+# frozen_string_literal: true
+
+module CustomField
+  extend ActiveSupport::Concern
+
+  class_methods do
+    def true_fields
+      where(value: HasCustomFields::Helpers::CUSTOM_FIELD_TRUE)
+    end
+  end
+end

app/models/group_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class GroupCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :group
 end
 

app/models/post_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class PostCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :post
 end
 

app/models/topic_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class TopicCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :topic
 end
 

app/models/user_custom_field.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class UserCustomField < ActiveRecord::Base
+  include CustomField
+
   belongs_to :user
 
   scope :searchable,