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/ClientInterface.php

@@ -0,0 +1,24 @@
+<?php
+
+namespace Kanboard\Core\Mail;
+
+/**
+ * Mail Client Interface
+ *
+ * @package  mail
+ * @author   Frederic Guillot
+ */
+interface ClientInterface
+{
+    /**
+     * Send a HTML email
+     *
+     * @access public
+     * @param  string  $email
+     * @param  string  $name
+     * @param  string  $subject
+     * @param  string  $html
+     * @param  string  $author
+     */
+    public function sendEmail($email, $name, $subject, $html, $author);
+}