MailService: Allow configuring AuthType.

This works around a broken mail server that claims to support CRAM-MD5,
but then actually does not.

app/classes/Framadate/Services/MailService.php

@@ -118,6 +118,7 @@ class MailService {
         $available_options = [
             'host' => 'Host',
             'auth' => 'SMTPAuth',
+            'authtype' => 'AuthType',
             'username' => 'Username',
             'password' => 'Password',
             'secure' => 'SMTPSecure',