Added new unique tag to all mailgun requests

refs: https://github.com/TryGhost/Ghost/issues/15725

- our users are having difficulties getting onboarded with mailgun
- we're adding an explicit and unique tag to all requests, to help mailgun detect when mail is being sent from Ghost

ghost/mailgun-client/lib/mailgun-client.js

@@ -71,7 +71,7 @@ module.exports = class MailgunClient {
                 messageData['v:email-id'] = message.id;
             }
 
-            const tags = ['bulk-email'];
+            const tags = ['bulk-email', 'ghost-email'];
             if (bulkEmailConfig?.mailgun?.tag) {
                 tags.push(bulkEmailConfig.mailgun.tag);
             }