API Security

- added APIAccess class to define access levels of API Endpoints (permissions)
- use "mailpoet_token" for all nonce (just as before)
- merged setupPublic/setupAdmin methods in API in order to avoid duplication
- check permission if access level is not all
- fixed ABSPATH check in some classes

lib/Mailer/Mailer.php

@@ -5,6 +5,8 @@ use MailPoet\Models\Setting;
 if(!defined('ABSPATH')) exit;
 require_once(ABSPATH . 'wp-includes/pluggable.php');
 
+require_once(ABSPATH . 'wp-includes/pluggable.php');
+
 class Mailer {
   public $mailer_config;
   public $sender;