Interpret X-Forwarded-For on app servers' nginx.

See:
    http://nginx.org/en/docs/http/ngx_http_realip_module.html#set_real_ip_from

(imported from commit adc4ebf46aefd1c71bda187d84519d8c31f6c590)

servers/puppet/modules/humbug/files/nginx/humbug-include/app

@@ -36,3 +36,7 @@ location / {
     fastcgi_buffers 1024 4k;
     fastcgi_max_temp_file_size 0;
 }
+
+real_ip_header X-Forwarded-For;
+# lb0.zulip.net elastic IP
+set_real_ip_from 54.213.41.54;