Add Site class reference

String 'Site' will not work with the current setup in SS4 anymore, so this check always fails, resulting in the preview not being hidden as intended.

src/Admin/MultisitesCmsMainExtension.php

@@ -104,7 +104,7 @@ public function updateEditForm($form) {
         $classNameField = $form->Fields()->dataFieldByName('ClassName');
         if ($classNameField) {
             $className = $classNameField->Value();
-            if ($className === 'Site') 
+            if ($className === Site::class) 
             {
             	$form->Fields()->removeByName(array(SilverStripeNavigator::class));
                 $form->removeExtraClass('cms-previewable');