Add webhooks-attributes to Sandstorm.

Thanks to xet7 !

sandstorm-pkgdef.capnp

@@ -244,6 +244,7 @@ const myCommand :Spk.Manifest.Command = (
     (key = "MATOMO_WITH_USERNAME", value="false"),
     (key = "BROWSER_POLICY_ENABLED", value="true"),
     (key = "TRUSTED_URL", value=""),
+    (key = "WEBHOOKS_ATTRIBUTES", value=""),
     (key = "SANDSTORM", value = "1"),
     (key = "METEOR_SETTINGS", value = "{\"public\": {\"sandstorm\": true}}")
   ]