Warning for VPN if UPnP is disabled

[dsimog01]

Users should be warned that VPN will not work in case that UPnP is disabled, unless they open the ports manually. This warning should be shown in the VPN tab of the dappmanager.

It is not possible to know if the ports are open from the dappmanager, so it should be enough with warning the users that if UPnP is disabled, they have to manually open the ports.