mentions: include mentioner in subject line.

The template looks like this:

Subject:

    #{creator_name} mentioned you in a comment on post ##{post_id}

Body:

    @#{creator_name} mentioned you in a \"comment\":/posts/#{post_id}#comment-#{id} on post ##{post_id}:

    [quote]
    #{DText.excerpt(body, "@"+user_name)}
    [/quote]

app/logical/d_text.rb

@@ -381,5 +381,12 @@ class DText
       })
     )
   end
+
+  # extract the first paragraph `needle` occurs in.
+  def self.excerpt(dtext, needle)
+    dtext = dtext.gsub(/\r\n|\r|\n/, "\n")
+    excerpt = ActionController::Base.helpers.excerpt(dtext, needle, separator: "\n\n", radius: 1, omission: "")
+    excerpt
+  end
 end