Add References: email header for better threading

Created by: foobacca

particularly for comments.

The References: header has a list of all message-id:s of messages that have come before it, so it allows threading even if the last message wasn't received by you (as it wouldn't be if you entered it yourself on the site).

Before this, if you enter a feedback on the site, then comments to the feedback won't find the message-id referenced in the In-Reply-To: header, so the comment email will start a new thread.