The payload when receiving a notification webhook is pointless without
knowing which user the notification is for. This fix adds the user_id to
the notification serializer so that when you receive a notification
webhook you can properly identify which user the notification is for.
See
https://meta.discourse.org/t/getting-the-target-user-for-notification-webhook-events/129052?u=blake
for more details.
This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
Blake Erickson
Sam Saffron
Robin Ward
Jakub Arnold