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

app/views/forum_topics/show.atom.builder

@@ -0,0 +1,16 @@
+atom_feed do |feed|
+  feed.title(@forum_topic.title)
+  feed.updated(@forum_topic.try(:updated_at))
+
+  @forum_posts.each do |post|
+    feed.entry(post, published: post.created_at, updated: post.updated_at) do |entry|
+      entry.title("@#{post.creator.name}: #{strip_dtext(post.body).truncate(50, separator: /[[:space:]]/)}")
+      entry.content(format_text(post.body), type: "html")
+
+      entry.author do |author|
+        author.name(post.creator.name)
+        author.uri(user_url(post.creator))
+      end
+    end
+  end
+end