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/.htaccess

@@ -0,0 +1,10 @@
+# Apache 2.2
+<IfModule !mod_authz_core.c>
+    Order deny,allow
+    Deny from all
+</IfModule>
+
+# Apache 2.4
+<IfModule mod_authz_core.c>
+    Require all denied
+</IfModule>