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/forum_post.rb

@@ -49,14 +49,7 @@ class ForumPost < ApplicationRecord
     end
   end
 
-  module ApiMethods
-    def html_data_attributes
-      super + [[:topic, :is_deleted?]]
-    end
-  end
-
   extend SearchMethods
-  include ApiMethods
 
   def self.new_reply(params)
     if params[:topic_id]