/comments.atom: add atom feed for comments.

app/views/comments/index.atom.builder

@@ -0,0 +1,24 @@
+atom_feed do |feed|
+  title = "Comments"
+  title += " by #{params[:search][:creator_name]}" if params.dig(:search, :creator_name).present?
+  title += " on #{params[:search][:post_tags_match]}" if params.dig(:search, :post_tags_match).present?
+
+  feed.title(title)
+  feed.updated(@comments.first.try(:updated_at))
+
+  @comments.each do |comment|
+    feed.entry(comment, published: comment.created_at, updated: comment.updated_at) do |entry|
+      entry.title("@#{comment.creator_name} on post ##{comment.post_id} (#{comment.post.humanized_essential_tag_string})")
+      entry.content(<<-EOS.strip_heredoc, type: "html")
+        <img src="#{comment.post.complete_preview_file_url}"/>
+
+        #{format_text(comment.body)}
+      EOS
+
+      entry.author do |author|
+        author.name(comment.creator_name)
+        author.uri(user_url(comment.creator))
+      end
+    end
+  end
+end