Model validation

[nemesifier]

We should add some basic model validation:

• [x] prevent saving an ip address which is not part of the specified subnet, test saving ip 192.168.1.1 on subnet 10.0.0.0/24, add a similar test for ipv6 too
• [x] how do you specify the network on a subnet? We need to validate that the prefix inserted is valid as well. I think you could add two fields to the subnet model: network and prefix, network can be a generic ip address field, and prefix an integer, before saving you need to validate these two fields (clean) by trying to create a network object with the ipaddress library, and if anything fails raise a ValidationError
• [x] what happens if a user tries to save the same ip twice?
• [x] what happens if a user tries to save the same subnet twice?
• [x] what happens if a user tries to create a subnet that overlaps with an existing one?

[nemesifier]

The last one is particularly tricky and we may not be able to implement it quickly with the current setup. Focus on addressing all the previous points in #11.