feat: use a Mailjet sub-account for travel lists

.env

@@ -44,6 +44,9 @@ MAILJET_API_KEY=
 MAILJET_API_SECRET=
 MAILJET_API_BASE_URL=https://api.mailjet.com/v3/REST
 MAILJET_DEFAULT_LIST_ID=
+# Sub-account for the travel mailing lists
+MAILJET_TRAVEL_MAILING_API_KEY=
+MAILJET_TRAVEL_MAILING_API_SECRET=
 # Development default for HTTP Basic Auth password; override in production secrets.
 MAILJET_WEBHOOK_BASIC_PASSWORD=secret
 APP_NEWSLETTER_CONFIRMATION_TTL_HOURS=1