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/Subscription.php

@@ -0,0 +1,24 @@
+<?php
+namespace MailPoet\Router\Endpoints;
+
+use MailPoet\Subscription as UserSubscription;
+
+if(!defined('ABSPATH')) exit;
+
+class Subscription {
+  const ENDPOINT = 'subscription';
+
+  static function confirm($data) {
+    $subscription = new UserSubscription\Pages('confirm', $data);
+    $subscription->confirm();
+  }
+
+  static function manage($data) {
+    $subscription = new UserSubscription\Pages('manage', $data);
+  }
+
+  static function unsubscribe($data) {
+    $subscription = new UserSubscription\Pages('unsubscribe', $data);
+    $subscription->unsubscribe();
+  }
+}