Add referral id to URLs in JS

[MAILPOET-2182]

assets/js/src/announcements/free_plan_announcement.jsx

@@ -1,5 +1,6 @@
 import React from 'react';
 import MailPoet from 'mailpoet';
+import addReferralId from 'referral_url_decorator.jsx';
 
 class FreePlanAnnouncement extends React.Component {
   constructor(props) {
@@ -32,7 +33,7 @@ class FreePlanAnnouncement extends React.Component {
           <p>{MailPoet.I18n.t('freePlanDescription')}</p>
           <a
             className="button-primary"
-            href="https://www.mailpoet.com/free-plan/"
+            href={addReferralId('https://www.mailpoet.com/free-plan/')}
             target="_blank"
             rel="noopener noreferrer"
           >