Add IPv6 support for isLocal checks (#1592)

* Add IPv6 support for isLocal checks

* Replace magic values like 127.0.0.1 and use our utility
* Support for IPv6 local adresses in IPv6 only or dual stack environments
    * Loopback [::1]
    * Site-Local fc00::/7
    * Link-local fe80::/10

* Introduce extra method for loopback addresses

* Use public IP for passMaxLogin check

* Use non-local IP addresses in test after change in verification

src/main/java/fr/xephi/authme/process/login/AsynchronousLogin.java

@@ -30,6 +30,7 @@ import fr.xephi.authme.settings.properties.EmailSettings;
 import fr.xephi.authme.settings.properties.HooksSettings;
 import fr.xephi.authme.settings.properties.PluginSettings;
 import fr.xephi.authme.settings.properties.RestrictionSettings;
+import fr.xephi.authme.util.InternetProtocolUtils;
 import fr.xephi.authme.util.PlayerUtils;
 import fr.xephi.authme.util.Utils;
 import org.bukkit.ChatColor;
@@ -325,8 +326,7 @@ public class AsynchronousLogin implements AsynchronousProcess {
         // Do not perform the check if player has multiple accounts permission or if IP is localhost
         if (service.getProperty(RestrictionSettings.MAX_LOGIN_PER_IP) <= 0
             || service.hasPermission(player, PlayerStatePermission.ALLOW_MULTIPLE_ACCOUNTS)
-            || "127.0.0.1".equalsIgnoreCase(ip)
-            || "localhost".equalsIgnoreCase(ip)) {
+            || InternetProtocolUtils.isLoopbackAddress(ip)) {
             return false;
         }