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/order_merchant_comment.txt

@@ -0,0 +1,13 @@
+
+[{shop_url}] 
+
+Dobrý deň {firstname} {lastname}, 
+
+Obdržali ste novú správu z obchodu {shop_name} k Vašej
+objednávke s číslom {order_name}.
+
+SPRÁVA: {message} 		 
+
+{shop_name} [{shop_url}] powered by
+PrestaShop(tm) [http://www.prestashop.com/] 
+