Add RFC4685 replies link to Atom feed

tildes/tildes/templates/topic_listing.atom.jinja2

@@ -16,6 +16,11 @@
             {% else %}
                 <link rel="alternate" href="{{ topic.permalink_absolute }}"/>
             {% endif %}
+            <link
+                rel="replies"
+                type="text/html"
+                thr:count="{{ topic.num_comments }}"
+                href="{{ topic.permalink_absolute }}"/>
             <content type="html"><![CDATA[
                 {% if topic.is_link_type %}
                     <p>Link URL: <a href="{{ topic.link }}">{{ topic.link }}</a></p>