apply Apache License

mail_templates.php

@@ -1,5 +1,18 @@
 <?php
-
+/**
+ * Copyright 2018 Matthias Kesler
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ * 
+ * http://www.apache.org/licenses/LICENSE-2.0
+ * 
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
 function send_mail($receiver, $subject, $body) {
 	include("config.php");
 	$headers = "From: " . $config["register_email"] . "\r\n"