Add external links for tasks with plugin api

app/Core/ExternalLink/ExternalLinkProviderNotFound.php

@@ -0,0 +1,15 @@
+<?php
+
+namespace Kanboard\Core\ExternalLink;
+
+use Exception;
+
+/**
+ * External Link Provider Not Found Exception
+ *
+ * @package  externalLink
+ * @author   Frederic Guillot
+ */
+class ExternalLinkProviderNotFound extends Exception
+{
+}