Statistics for Form Subscriptions

- added statistics_forms table
- added corresponding model to record stats
- record stats whenever someone subscribes via a form

lib/Router/Subscribers.php

@@ -8,6 +8,7 @@ use MailPoet\Models\SubscriberCustomField;
 use MailPoet\Models\Segment;
 use MailPoet\Models\Setting;
 use MailPoet\Models\Form;
+use MailPoet\Models\StatisticsForms;
 use MailPoet\Util\Url;
 
 if(!defined('ABSPATH')) exit;
@@ -92,6 +93,11 @@ class Subscribers {
     $errors = $subscriber->getErrors();
     $result = ($errors === false && $subscriber->id() > 0);
 
+    // record form statistics
+    if($result === true && $form !== false && $form->id > 0) {
+      StatisticsForms::record($form->id);
+    }
+
     // get success message to display after subscription
     $form_settings = (
       isset($form->settings)