Use configured default page also for IPv6

Just a small check for the ipv6 variable, similar to _listen.conf,
so that the configured default page is also delivered on ipv6 requests.

backend/templates/default.conf

@@ -6,6 +6,11 @@
 {%- else %}
 server {
   listen 80 default;
+{% if ipv6 -%}
+  listen [::]:80;
+{% else -%}
+  #listen [::]:80;
+{% endif %}
   server_name default-host.localhost;
   access_log /data/logs/default_host.log combined;
 {% include "_exploits.conf" %}