Add block IP address and subnet validation

[codeling]

When entering a subnet or IP address for blocking, bfstop should make sure that it the entered string fulfills the following criteria:

• It is in a format that represents a valid IP v4 or v6 address or subnet .
• the given range or IP address should not block the administrator herself.

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