/forum_topics.atom: add atom feed for forum topics.

app/views/forum_topics/index.atom.builder

@@ -0,0 +1,16 @@
+atom_feed do |feed|
+  feed.title("Forum Topics")
+  feed.updated(@forum_topics.first.try(:updated_at))
+
+  @forum_topics.each do |topic|
+    feed.entry(topic, published: topic.created_at, updated: topic.updated_at) do |entry|
+      entry.title("[#{topic.category_name}] #{topic.title}")
+      entry.content(format_text(topic.original_post.body), type: "html")
+
+      entry.author do |author|
+        author.name(topic.creator.name)
+        author.uri(user_url(topic.creator_id))
+      end
+    end
+  end
+end