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/voucher.txt

@@ -0,0 +1,16 @@
+
+[{shop_url}] 
+
+Dobrý deň {firstname} {lastname}, 
+
+Vytvorili sme Vám nový kupón vďaka objednávke s číslom
+{order_name}.
+
+KÓD ZĽAVOVÉHO KUPÓNU: {voucher_num} v hodnote {voucher_amount}
+
+Jednoducho skopírujte tento kód a vložte ho do Vašej ďalšej
+objednávky v našom obchode. 		 
+
+{shop_name} [{shop_url}] powered by
+PrestaShop(tm) [http://www.prestashop.com/] 
+