Feature: join servers based on their invite code

This removes the need to know a server IP to join it. Invite codes
are small (~7 characters) indentifiers for servers, which can be
exchanged with other players to join the servers.

src/network/core/game_info.cpp

@@ -141,7 +141,11 @@ void FillStaticNetworkServerGameInfo()
  */
 const NetworkServerGameInfo *GetCurrentNetworkServerGameInfo()
 {
-	/* Client_on is used as global variable to keep track on the number of clients. */
+	/* These variables are updated inside _network_game_info as if they are global variables:
+	 *  - clients_on
+	 *  - invite_code
+	 * These don't need to be updated manually here.
+	 */
 	_network_game_info.companies_on  = (byte)Company::GetNumItems();
 	_network_game_info.spectators_on = NetworkSpectatorCount();
 	_network_game_info.game_date     = _date;