Bug#865520: calypso: does not listen on IPv6 address

[Roland Hieber]

For the record, an obvious workaround is to put calypso behind a reverse
proxy, for example with nginx:

server {
        listen 80 default_server;
        listen [::]:80 default_server ipv6only=on;
        listen 443 default_server ssl;
        listen [::]:443 default_server ipv6only=on ssl;
# [...]
        location /calendar {
                proxy_set_header Host $host;
                proxy_buffering off;
                proxy_pass https://localhost:5233/calendar;
        }
}

 - Roland