add EnablePreviewModeBanner in config (#8834)

Signed-off-by: Saturnino Abril <saturnino.abril@gmail.com>

config/default.json

@@ -177,6 +177,7 @@
         "EnableEmailBatching": false,
         "EmailBatchingBufferSize": 256,
         "EmailBatchingInterval": 30,
+        "EnablePreviewModeBanner": true,
         "SkipServerCertificateVerification": false,
         "EmailNotificationContentsType": "full",
         "LoginButtonColor": "",