/forum_topics/$id: fix atom feed discovery link.

> Found an issue with the forum topic ATOM discovery link. Currently the
> link is visible to feed detectors, but they are unable to add it since
> the link provided is invalid.
>
> <link rel="alternate" type="application/atom+xml" title="Artist tagging
> help" href="http://danbooru.donmai.us/forum_topics.8199" />

app/views/forum_topics/show.html.erb

@@ -43,6 +43,8 @@
 <% end %>
 
 <%= content_for(:html_header) do %>
+  <%= auto_discovery_link_tag(:atom, {format: :atom}, {title: @forum_topic.title}) %>
+
   <script>
     $(function() {
       $("#new-response-link").click(function(e) {
@@ -57,5 +59,3 @@
     });
   </script>
 <% end %>
-
-<% content_for(:html_header, auto_discovery_link_tag(:atom, forum_topics_url(@forum_topic, :atom), title: @forum_topic.title)) %>