MINOR add a hidden field to attach the subsite ID, rather than relying on the session (from r91014)

git-svn-id: svn://svn.silverstripe.com/silverstripe/open/modules/cms/branches/2.4@96808 467b73ca-7a2a-4603-9d3b-597d59a354a9

code/CMSMain.php

@@ -1113,6 +1113,8 @@ JS;
 			new DropdownField("PageType", "", $pageTypes, 'Page')
 		);
 		
+		$this->extend('updatePageOptions', $fields);
+		
 		$actions = new FieldSet(
 			new FormAction("addpage", _t('CMSMain.GO',"Go"))
 		);