feat: allowing up to /20 networks

[Tbaile]

• VPN network can now be anywhere from /20 to /24, instead of only a class C. New installs get a random /20 inside 172.16.0.0/12. Existing /24 controllers keep their network.
• Added a set-vpn-network action. It stops the controller, removes the tun and saves the new network, then runs configure-module again, which recreates the tun and the firewall rules. Hidden from the UI due to the possibility to overlap networks and create issues in the node.