Add referral program invitation and voucher email templates

- Created `referralprogram-invitation.html.twig` for sending invitations to friends to join the referral program.
- Created `referralprogram-voucher.html.twig` for notifying users about their voucher after referring a friend.
- Both templates include responsive design and utilize the modern email layout.
- Added translation support for various text elements within the templates.

mails/themes/classic/core/newsletter.html.twig

@@ -0,0 +1,14 @@
+{% extends '@MailThemes/classic/components/layout.html.twig' %}
+
+{% block content %}
+<tr>
+  <td align="center" class="titleblock">
+    <span class="title">{{ 'Hi {firstname} {lastname},'|trans({}, 'Emails.Body', locale) }}</span>
+  </td>
+</tr>
+<tr>
+  <td class="linkbelow">
+    {message}
+  </td>
+</tr>
+{% endblock %}