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/guest_to_customer.txt

@@ -0,0 +1,19 @@
+
+[{shop_url}] 
+
+Dobrý deň {firstname} {lastname}, 
+
+Váš účet návštevníka v obchode {shop_name} bol zmenený na
+zákaznícky účet. 
+
+E-MAIL: {email} 		 
+
+Pri zdieľaní týchto prihlasovacích údajov s ostatnými buďte
+prosím opatrný. 
+
+Môžete sa prihlásiť do svojho zákazníckeho účtu v našom
+obchode: {shop_url} 
+
+{shop_name} [{shop_url}] powered by
+PrestaShop(tm) [http://www.prestashop.com/] 
+