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/forum_topic_policy.rb

@@ -37,4 +37,8 @@ class ForumTopicPolicy < ApplicationPolicy
       ([:is_sticky, :is_locked, :min_level] if moderate?)
     ].compact.flatten
   end
+
+  def html_data_attributes
+    super + [:is_read?]
+  end
 end