Set “Reply-To” in diagram comment notification mails.

Currently, diagram comment notification mails are sent with From: admin@strathspey.org. This is a problem because whoever answers them will hit “Reply”, and then the answer will go to me instead of who ever originally submitted the comment.

Obviously we can't forge e-mails from the original sender (From: original-sender@wherever.net) because that will cause problems with things like SPF. Therefore we should add Reply-To: original-sender@wherever.net to these messages. That way at least if the message still goes to admin@strathspey.org, it's the fault of whatever crappy e-mail software the diagram person is using.

(The same applies to crib comment notification mails.)