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,...)

tests/unit/API/CustomFieldsTest.php

@@ -1,6 +1,6 @@
 <?php
 
-use \MailPoet\API\CustomFields;
+use \MailPoet\API\Endpoints\CustomFields;
 use \MailPoet\Models\CustomField;
 
 class CustomFieldsTest extends MailPoetTest {