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/policies/wiki_page_policy.rb

@@ -18,4 +18,8 @@ class WikiPagePolicy < ApplicationPolicy
   def api_attributes
     super + [:category_name]
   end
+
+  def html_data_attributes
+    super + [:category_name]
+  end
 end