Add missing configuration for hedgedoc websocket support

The websocket support always 400'd as upgrade headers where missing for
/socket.io

roles/hedgedoc/templates/nginx.d.conf.j2

@@ -2,6 +2,11 @@ upstream hedgedoc {
     server localhost:3000;
 }
 
+map $http_upgrade $connection_upgrade {
+        default upgrade;
+        ''      close;
+}
+
 server {
     listen       80;
     listen       [::]:80;
@@ -43,5 +48,7 @@ server {
        proxy_set_header X-Real-IP $remote_addr;
        proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
        proxy_set_header X-Forwarded-Proto $scheme;
+       proxy_set_header Upgrade $http_upgrade;
+       proxy_set_header Connection $connection_upgrade;
    }
 }