Added 'bulk-email' tag to bulk messages sent with mailgun

no issue

- helps distinguish bulk vs transactional email when both are using the same mailgun account

core/server/services/bulk-email/index.js

@@ -55,7 +55,7 @@ module.exports = {
 
                 if (bulkEmailConfig && bulkEmailConfig.mailgun && bulkEmailConfig.mailgun.tag) {
                     Object.assign(messageData, {
-                        'o:tag': bulkEmailConfig.mailgun.tag
+                        'o:tag': [bulkEmailConfig.mailgun.tag, 'bulk-email']
                     });
                 }