RECAP alerts need to support either filing alerts or case alerts

[mlissner]

I've been using the alerts for a while now and one thing I've noticed and heard from other beta testers is that the alerts have two primary purposes:

1. Learning about new filings. I use this, for example, with a weekly alert, to know if CourtListener shows up in PACER.

2. Learning about new cases. I use this, for example, to learn about new lawsuits against the Trump admin.

I think the easiest way to show this to the user is to make it a big choice on the alert modal, but I'd love for @s-taube to think about this some more and maybe do a little research on it.

Here's a notional design, just to express the idea, by putting two buttons at the bottom:

On the back end, I think we do this by triggering alerts as normal, but then keeping a redis set of dockets triggered for each alert in addition to (or instead of) the list of filings for each alert. Then, if there's a second filing for a case that triggers the alert, we can just drop it, like we do now for filings.