Add a webhook handler for RECAP alerts

[mlissner]

Soon, RECAP will have alerts which send webhook events: freelawproject/courtlistener#612

When that is live, we should set up a webhook handler in the bot that catches those events and then automatically subscribes to related cases.

For example, today Rudy Giuliani was made to pay $148M to election workers. He'll be filing for bankruptcy soon. What if I could create an alert for that bankruptcy today, before he files it, and then when that alert triggers in a few months, we just automatically subscribe to it.

Sounds pretty neat to me.

[mlissner]

I think this will have to work by my inclusion of a magic word in the description of the alert. Like, maybe I have to name the alert bigcases-autosubscribe so that the bot know what to do when the event lands. Hmmmmmm....