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/themes/classic/assets/rtl.css

@@ -0,0 +1,4 @@
+body{
+  direction: rtl;
+  font-family: Tahoma, "Open-sans", sans-serif;
+}