#784 Perform bypass purge permission check with OfflinePlayer objects

- Move permission check inside PurgeTask to perform it with OfflinePlayer objects instead of lowercase names
- Move purge members into child "purge" package
- Unify online and offline default permission behavior in DefaultPermission

src/main/java/fr/xephi/authme/permission/handlers/VaultHandler.java

@@ -50,7 +50,7 @@ public class VaultHandler implements PermissionHandler {
     }
 
     @Override
-    public boolean hasPermission(String name, PermissionNode node) {
+    public boolean hasPermissionOffline(String name, PermissionNode node) {
         return vaultProvider.has("", name, node.getNode());
     }