User registration webhook (per-application)

[zomglings]

Each application should have the option of registering a webhook endpoint to which we send an event every time a user registers.

The event can have the following schema:

{
    "event_type": "user_registration",
    "user_id": "<id>"
}

[zomglings]

This is useful for applications external to Bugout.

[zomglings]

We will use it in Moonstream like this:

1. Moonstream application will register webhook URL with brood: https://api.moonstream.to/webhooks.
2. Every time a user registers, Brood will `POST https://api.moonstream.to -H "Content-Type: application/json" -d '{"event_type": "user_registration", "user_id": "<user_id>"}'
3. Moonstream, whenever it receives this webhook, will send the user a welcome email, set up the user's default subscriptions, and set a state in the database indicating that user onboarding was set up.
4. (Not relevant to Brood, but worth writing for @peersky) Moonstream frontend will check user's onboarding status. If onboarding has not even been set up, it will display a message to the user indicating that we are setting things up for them. If onboarding has been set up but user has not interacted with it, we will show them onboarding flow (including default subscriptions). Once user finishes onboarding, frontend will send a message to backend saying that onboarding is complete, and backend will store this in the user's onboarding state.