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/Client.php

@@ -0,0 +1,96 @@
+<?php
+
+namespace Kanboard\Core\Mail;
+
+use Pimple\Container;
+use Kanboard\Core\Base;
+
+/**
+ * Mail Client
+ *
+ * @package  mail
+ * @author   Frederic Guillot
+ */
+class Client extends Base
+{
+    /**
+     * Mail transport instances
+     *
+     * @access private
+     * @var \Pimple\Container
+     */
+    private $transports;
+
+    /**
+     * Constructor
+     *
+     * @access public
+     * @param  \Pimple\Container   $container
+     */
+    public function __construct(Container $container)
+    {
+        parent::__construct($container);
+        $this->transports = new Container;
+    }
+
+    /**
+     * Send a HTML email
+     *
+     * @access public
+     * @param  string  $email
+     * @param  string  $name
+     * @param  string  $subject
+     * @param  string  $html
+     * @return EmailClient
+     */
+    public function send($email, $name, $subject, $html)
+    {
+        $this->container['logger']->debug('Sending email to '.$email.' ('.MAIL_TRANSPORT.')');
+
+        $start_time = microtime(true);
+        $author = 'Kanboard';
+
+        if ($this->userSession->isLogged()) {
+            $author = e('%s via Kanboard', $this->user->getFullname($this->session['user']));
+        }
+
+        $this->getTransport(MAIL_TRANSPORT)->sendEmail($email, $name, $subject, $html, $author);
+
+        if (DEBUG) {
+            $this->logger->debug('Email sent in '.round(microtime(true) - $start_time, 6).' seconds');
+        }
+
+        return $this;
+    }
+
+    /**
+     * Get mail transport instance
+     *
+     * @access public
+     * @param  string  $transport
+     * @return EmailClientInterface
+     */
+    public function getTransport($transport)
+    {
+        return $this->transports[$transport];
+    }
+
+    /**
+     * Add a new mail transport
+     *
+     * @access public
+     * @param  string  $transport
+     * @param  string  $class
+     * @return EmailClient
+     */
+    public function setTransport($transport, $class)
+    {
+        $container = $this->container;
+
+        $this->transports[$transport] = function() use ($class, $container) {
+            return new $class($container);
+        };
+
+        return $this;
+    }
+}