One cancellation dialog should be sufficient

In the latest release version, there appear two cancellation dialogs if the user wants to turn off VPN. One cancellation dialog is raised by DisconnectVPN Activity which is part of ics-openvpn. The other is triggered by Bitmask's Dashboard Activity. We should suppress the DisconnectVPN cancellation dialog in favor of the Dashboard's dialog. The latter includes a more useful warning: "In order to avoid leaking your personal information, please close your browser and start a private window after disconnecting the Encrypted VPN Internet Access. Thanks." There needs to be done minor changes in ics-openvpn to implement that behavior.