Add logic to easily switch to "whitelist mode" for registering servers

[GeckoEidechse]

For situations where someone is actively messing with registering game servers, we could easily shut them out by just temporarily switching to a whitelist mode, where only a whitelisted subset of IP addresses would be allowed to register. These addresses would belong to known major hosters as well as trusted persons that would be able to take the bulk of the playerbase until server registration is restored to public again.

IP addresses as identifiers should be fine as an attacker would need to be able to both spoof IP addresses (easy) and be able to respond from those spoofed addresses (requires control of the network, obviously difficult).

The whitelisted IPs could probably just be stored in a text file, with one line per IP address.