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/sk/productoutofstock.txt

@@ -0,0 +1,16 @@
+
+[{shop_url}] 
+
+Dobrý deň, 
+
+Množstvo na sklade je momentálne menšie ako špecifikované
+minimum {last_qty}.
+
+NA SKLADE ZOSTÁVA: {qty}
+
+Odporúčame otvoriť stránku produktu v administrácii a doplniť
+inventár. 		 
+
+{shop_name} [{shop_url}] powered by
+PrestaShop(tm) [http://www.prestashop.com/] 
+