Added config to send bulk email in testmode

- mailgun has a testmode flag we can use to get email to be accepted but not delivered
- this is useful for developers testing general bulk email code - not for users - so it is only available via config

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

@@ -104,6 +104,12 @@ module.exports = {
                 });
             }
 
+            if (bulkEmailConfig && bulkEmailConfig.mailgun && bulkEmailConfig.mailgun.testmode) {
+                Object.assign(batchData, {
+                    'o:testmode': true
+                });
+            }
+
             const messageData = Object.assign({}, message, batchData);
 
             // Rename plaintext field to text for Mailgun