Add referral id URL decorator

[MAILPOET-2182]

lib/Referrals/UrlDecorator.php

@@ -0,0 +1,30 @@
+<?php
+
+namespace MailPoet\Referrals;
+
+use MailPoet\Settings\SettingsController;
+use MailPoet\WP\Functions as WPFunctions;
+
+if (!defined('ABSPATH')) exit;
+
+class UrlDecorator {
+
+  /** @var WPFunctions */
+  private $wp;
+
+  /** @var SettingsController */
+  private $settings;
+
+  function __construct(WPFunctions $wp, SettingsController $settings) {
+    $this->wp = $wp;
+    $this->settings = $settings;
+  }
+
+  function decorate($url) {
+    $referral_id = $this->settings->get(ReferralDetector::REFERRAL_SETTING_NAME, null);
+    if ($referral_id === null) {
+      return $url;
+    }
+    return $this->wp->addQueryArg('ref', $referral_id, $url);
+  }
+}