support scheduled crawls

[cldellow]

This will require schema changes plus something cron-esque.

[simonw]

For a while I've been thinking that Datasette could benefit from its own cron-like mechanism - maybe as a datasette-cron plugin.

I've not got any further than thinking it might be useful though!

This library looked like it might be a good fit: https://schedule.readthedocs.io/en/stable/background-execution.html