Make mail transports pluggable and move integrations to plugins

- Postmark: https://github.com/kanboard/plugin-postmark
- Mailgun: https://github.com/kanboard/plugin-mailgun
- Sendgrid: https://github.com/kanboard/plugin-sendgrid

app/Core/Mail/Transport/Sendmail.php

@@ -0,0 +1,25 @@
+<?php
+
+namespace Kanboard\Core\Mail\Transport;
+
+use Swift_SendmailTransport;
+
+/**
+ * PHP Mail Handler
+ *
+ * @package  transport
+ * @author   Frederic Guillot
+ */
+class Sendmail extends Mail
+{
+    /**
+     * Get SwiftMailer transport
+     *
+     * @access protected
+     * @return \Swift_Transport
+     */
+    protected function getTransport()
+    {
+        return Swift_SendmailTransport::newInstance(MAIL_SENDMAIL_COMMAND);
+    }
+}