Enable IPv6 on iec

[jhenstridge]

At present, IPv6 is disabled on our server:

If we flip this switch, there's a few things we'd need to do:

1. Update DNS records to add AAAA records, and make sure SPF records cover v6 address
2. Check that nginx and postfix are accessible via IPv6
3. Check that outgoing mail over v6 works
4. Other services?