Add StringTemplateTrait under old namespace for easier migration.

src/View/Helper/StringTemplateTrait.php

@@ -0,0 +1,25 @@
+<?php
+/**
+ * CakePHP(tm) : Rapid Development Framework (http://cakephp.org)
+ * Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ *
+ * Licensed under The MIT License
+ * For full copyright and license information, please see the LICENSE.txt
+ * Redistributions of files must retain the above copyright notice.
+ *
+ * @copyright     Copyright (c) Cake Software Foundation, Inc. (http://cakefoundation.org)
+ * @link          http://cakephp.org CakePHP(tm) Project
+ * @since         3.0.0
+ * @license       http://www.opensource.org/licenses/mit-license.php MIT License
+ */
+
+class_alias('Cake\View\StringTemplateTrait', 'Cake\View\Helper\StringTemplateTrait');
+
+trigger_error(
+	sprintf(
+		'%s has been moved. Use %s instead.',
+		'Cake\View\Helper\StringTemplateTrait',
+		'Cake\View\StringTemplateTrait'
+	),
+	E_USER_WARNING
+);