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/shipped.txt

@@ -0,0 +1,19 @@
+
+[{shop_url}] 
+
+Dobrý deň {firstname} {lastname},
+
+Vaša objednávka bola odoslaná 
+
+Vaša objednávka s číslom {order_name} bola odoslaná.
+
+Ďakujeme za nákup v obchode {shop_name}! 		 
+
+Podrobnosti o objednávke spolu s Vašou faktúrou nájdete vo
+Vašom zákazníckom "Účte" [{my_account_url}] po
+kliknutí na "História objednávok"
+[{history_url}]. 
+
+{shop_name} [{shop_url}] powered by
+PrestaShop(tm) [http://www.prestashop.com/] 
+