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/pl/_partials/order_conf_cart_rules.txt

@@ -0,0 +1,3 @@
+{foreach $list as $cart_rule}
+	{$cart_rule['voucher_name']}  {$cart_rule['voucher_reduction']}
+{/foreach}