enable dkim on mailing lists

[irabinovitch]

To improve delivery of our messages we should enable DKIM on lists.linuxfests.org. We definitely want to sign messages from phpList. I'm not sure what the best practice is around mailman though, since messages are often already signed by the original sender.

[irabinovitch]

Currently handled by mailgun at some point we probably want to move to SES or move back to handling this on our own boxes.