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/pl/credit_slip.txt

@@ -0,0 +1,15 @@
+ 
+
+{shop_url}
+
+Witaj {firstname} {lastname},
+
+Numer zamówienia {order_name} - Pokwitowanie - korekta kredytowa
+
+W Twoim imieniu został wygenerowany kupon na zamówienie o numerze {order_name}.
+
+Przejrzyj ten rachunek i pobierz fakturę w naszym sklepie, przejdź do sekcji[Moje pokwitowania - korekty płatności]({history_url}) konta klienta.
+
+[{shop_name}]({shop_url})
+
+Powered by [PrestaShop](https://www.prestashop.com/?utm_source=marchandprestashop&utm_medium=e-mail&utm_campaign=footer_1-7)