Added API/Endpoint abstract class

- (re)Added Endpoints folder to both API and Router
- fixed syntax in namespaces
- xhr.responseJSON is returned to the fail()
- fixed Router endpoints (view in browser, cron,...)

lib/Router/Endpoints/ViewInBrowser.php

@@ -0,0 +1,16 @@
+<?php
+namespace MailPoet\Router\Endpoints;
+
+use MailPoet\Newsletter\ViewInBrowser as NewsletterViewInBrowser;
+
+if(!defined('ABSPATH')) exit;
+
+class ViewInBrowser {
+  const ENDPOINT = 'view_in_browser';
+  const ACTION_VIEW = 'view';
+
+  static function view($data) {
+    $viewer = new NewsletterViewInBrowser($data);
+    $viewer->view();
+  }
+}