Servers added by IPv6 IP do not persist after relaunching the game

When adding an IPv6 server by its IP, the server does not appear in the server list after relaunching the game.

This was initially reported unofficially nearly a year ago.

The cause of the bug appears to be that the `:` character is used to separate server name, address and port in `external_servers.txt`. A typical IPv4 server looks like this:
2:dev:192.168.0.9:19132:1515091629

whereas an IPv6 server looks like this:
15:laptop_ipv6:fe80::f4c8:1278:cbdc:f35a:19132:1515522225

The obvious problem being that the IPv6 address is then parsed incorrectly due to the colons.