Add daily/weekly digest feature for docket alerts

[mlissner]

We have a nice feature for search alerts, where you can get them just once/day or once/week. A couple times now I've seen people requesting something like this or otherwise being sad at the number and deluge of alerts in their inbox.

We could fix this in a few ways:

1. Add an option to "Send as daily digest"

   If we do this, we could simply send the alerts as a single digest for each case at the end of each week/day. This would send one digest email per case, which would all land in your inbox at the same time. Meh?

2. Add an options to "Send in daily digest" or "Send in weekly digest"

   If we do this, we'd create one digest with all of the updates for subscribed cases, and send one email per user per day/week.

3. Allow custom digests and allow people to "Add alert to digest", with a dropdown to select the one they want.

   If we do this, people can create various digests, set a schedule for each digest, and then send all the alerts for a given digest according to that rate.

   This is maximally configurable, but also maximally complex from a UX and dev perspective.

I think I lean towards option 2. It's relatively simple to develop, and would solve this problem in a single email. This feels a bit like it should depend on #4271