FIX: Sync preload key format for category topic lists

The server and client used two different formats for preload keys. The
server was using 'topic_list_c/SLUG/l/latest', but the client was using
'topic_list_c/SLUG/ID/l/latest'.

This commit is an addition to 374534f00e.

app/models/topic_list.rb

@@ -67,7 +67,7 @@ class TopicList
 
   def preload_key
     if @category
-      "topic_list_#{@category.url.sub(/^\//, '')}/l/#{@filter}"
+      "topic_list_#{@category.url.sub(/^\//, '')}/#{@category.id}/l/#{@filter}"
     else
       "topic_list_#{@filter}"
     end