FIX: Stop register_topic_list_preload_user_ids from breaking old versions (#160)

We need a proper fix, but this stops the bleeding for now.
Wrapped code related to `register_topic_list_preload_user_ids` in
a `respond_to?(:register_topic_list_preload_user_ids)` block because
this API is not available in beta or stable versions of Discourse.

plugin.rb

@@ -695,6 +695,7 @@ SQL
       .count
   end
 
+  if respond_to?(:register_topic_list_preload_user_ids)
     class ::Topic
       attr_accessor :accepted_answer_user_id
     end
@@ -744,4 +745,5 @@ SQL
 
       prepend AddSolvedToTopicPostersSummary
     end
+  end
 end