Add whitelist IP address and subnet validation

[codeling]

When entering a subnet or IP address for blocking, bfstop should make sure that it the entered string is in a format that represents a valid IP v4 or v6 address or subnet .

There should also be a possibility to add the administrator's own IP address to the whitelist with a single click.

In addition, for subnets it could be useful to show a hint on the subnet that is whitelisted, possibly in the form of start and end address, and the number of addresses possibly contained in this subnet.