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/modules/ps_emailsubscription/newsletter_conf.html.twig

@@ -0,0 +1,33 @@
+{% extends '@MailThemes/classic/components/layout.html.twig' %}
+
+{% block content %}
+<tr>
+  <td align="center" class="titleblock">
+    <font size="2" face="{{ languageDefaultFont }}Open-sans, sans-serif" color="#555454">
+      <span class="title">{{ 'Hi,'|trans({}, 'Emails.Body', locale)|raw }}</span>
+    </font>
+  </td>
+</tr>
+<tr>
+  <td class="space_footer">&nbsp;</td>
+</tr>
+<tr>
+  <td class="box" style="border:1px solid #D6D4D4;">
+    <table class="table">
+      <tr>
+        <td width="10">&nbsp;</td>
+        <td>
+          <font size="2" face="{{ languageDefaultFont }}Open-sans, sans-serif" color="#555454">
+            <span>
+              <strong>
+                {{ 'Thank you for subscribing to our newsletter.'|trans({}, 'Emails.Body', locale)|raw }}
+              </strong>
+            </span>
+          </font>
+        </td>
+        <td width="10">&nbsp;</td>
+      </tr>
+    </table>
+  </td>
+</tr>
+{% endblock %}