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/post_policy.rb

@@ -95,4 +95,8 @@ class PostPolicy < ApplicationPolicy
     attributes -= [:fav_string] if !user.is_moderator?
     attributes
   end
+
+  def html_data_attributes
+    super + [:has_large?, :current_image_size]
+  end
 end