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/header.html.twig

@@ -0,0 +1,7 @@
+<tr>
+  <td align="center" class="logo" style="border-bottom: 4px solid #333333">
+    <a title="{shop_name}" href="{shop_url}" test="{shop_url}">
+      <img src="{shop_logo}" alt="{shop_name}" />
+    </a>
+  </td>
+</tr>