models: move html_data_attributes to policies.

Move html_data_attributes definitions from models to policies. Which
attributes are permitted as data-* attributes is a view level concern
and should be defined on the policy level, not the model level. Models
should be agnostic about how they're used in views.

app/models/wiki_page.rb

@@ -105,14 +105,7 @@ class WikiPage < ApplicationRecord
     end
   end
 
-  module ApiMethods
-    def html_data_attributes
-      super + [:category_name]
-    end
-  end
-
   extend SearchMethods
-  include ApiMethods
 
   def validate_rename
     return unless title_changed?