Network deletion doesn't trigger DHCP range deletion

[bcollet]

If a network is split in smaller subnets, but has a DHCP range larger than the smaller subnets it covers, it is not possible to edit or delete it any more.

Steps to reproduce:

1. Create a network (say a /24).
2. Create a DHCP range covering that network.
3. Delete the network.
4. Create several small networks in the former network.

The expected behaviour would be for the DHCP range to be deleted alongside the network (or for a warning to be presented to the user for him to chose what to do).

[pdav]

To solve this problem, the proposed solution is to add a foreign key from the dhcprange to the network id.
This way, network deletion is forbidden while a dhcprange references it.