Better subnet validation, static DHCP leases outside of dynamic IP range

[ainar-g]

Before #2541 users could add static DHCP leases both outside of the range for dynamically assigned IP addresses and outside of the subnet address space. Currently, we only allow adding static DHCP within the dynamic IP address range. The original behaviour was almost certainly too lax, but the new one is probably too restrictive. What we should do instead is allowing people to assign a static DHCP lease outside of the dynamic IP range, but only as long as the IP address is still within the subnet.