Unblock ports in the firewall on NixOS

This article was originally published at https://gist.github.com/joepie91/ce9267788fdcb37f5941be5a04fcdd0f. 

The firewall is enabled by default. This is how you open a port:

{
  # ...
  networking = {
    # ...
    
    firewall = {
      allowedTCPPorts = [ 24800 ];
    };
  };
  # ...
}