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

doc/plugin-overrides.markdown

@@ -0,0 +1,36 @@
+Plugin Overrides
+================
+
+Override HTTP Content Security Policy
+-------------------------------------
+
+If you would like to replace the default HTTP Content Security Policy header, you can use the method `setContentSecurityPolicy()`:
+
+```php
+<?php
+
+namespace Kanboard\Plugin\Csp;
+
+use Kanboard\Core\Plugin\Base;
+
+class Plugin extends Base
+{
+    public function initialize()
+    {
+        $this->setContentSecurityPolicy(array('script-src' => 'something'));
+    }
+}
+```
+
+Template Overrides
+------------------
+
+Any templates defined in the core can be overrided. By example, you can redefine the default layout or change email notifications.
+
+Example of template override:
+
+```php
+$this->template->setTemplateOverride('header', 'theme:layout/header');
+```
+
+The first argument is the original template name and the second argument the template to use as replacement.