webserver: Enable HTTP/2

webserver/templates/sites/hostnamerewrite.conf

 server {
     listen 80;
-    listen 443 ssl;
+    listen 443 ssl http2;
     server_name {{server.forward_hostnames.hostnames|default(server.forward_hostnames)|join(" ")}};
     error_log /var/log/nginx/rewrite-error-{{server.forward_hostnames.hostnames|default(server.forward_hostnames)|first}}.log;
     {% if server.buffer_access_log|default(webserver_buffer_access_log|default(False)) %}