Inline generateToken for clarity

The only thing Security::generateToken was providing was a default value
 for the $action, which created a pattern of using the same $action
 everywhere, which may not be the best way to go.

 Since it was essentially a wrapper for WP's built-in nonce functions,
 it seemed clearer to use those functions directly to be more explicit
 about how we're handling tokens.

[MAILPOET-2030]

mailpoet/lib/Form/DisplayFormInWPContent.php

@@ -7,7 +7,6 @@ use MailPoet\Config\Renderer as TemplateRenderer;
 use MailPoet\Entities\FormEntity;
 use MailPoet\Subscribers\SubscribersRepository;
 use MailPoet\Subscribers\SubscriberSubscribeController;
-use MailPoet\Util\Security;
 use MailPoet\WP\Functions as WPFunctions;
 
 class DisplayFormInWPContent {
@@ -188,7 +187,7 @@ class DisplayFormInWPContent {
     }
 
     // generate security token
-    $templateData['token'] = Security::generateToken();
+    $templateData['token'] = $this->wp->wpCreateNonce('mailpoet_token');
 
     // add API version
     $templateData['api_version'] = API::CURRENT_VERSION;