rewrite topics routes to include id

app/templates/topics/topic.html

@@ -16,7 +16,7 @@
   {%- if is_logged_in() and get_active_user().can_post_to_topic(topic) -%}
   <a href="{{url_for('threads.new', topic_id=topic.id)}}" class="linkbutton">New thread</a>
   {%- endif -%}
-  <a href="{{url_for('topics.feed', slug=topic.slug)}}" class="linkbutton rss">Subscribe via RSS</a>
+  <a href="{{url_for('topics.feed', topic_id=topic.id)}}" class="linkbutton rss">Subscribe via RSS</a>
   <form method="GET">
     <select name="sort_by">
       <option value="activity"{% if sort_by == 'activity' %}selected{% endif %}>Sorted by activity</option>