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/components/footer.html.twig

@@ -0,0 +1,8 @@
+<tr>
+  <td class="space_footer">&nbsp;</td>
+</tr>
+<tr>
+  <td class="footer" style="border-top: 4px solid #333333">
+    <span>{{ '<a href="{shop_url}">{shop_name}</a> powered by <a href="{prestashop_url}">PrestaShop™</a>'|trans({'{prestashop_url}': 'https://www.prestashop.com/?utm_source=marchandprestashop&utm_medium=e-mail&utm_campaign=footer_1-7'}, 'Emails.Body', locale)|raw }}</span>
+  </td>
+</tr>