HOTFIX: Rewrite Marin data scraper to use Socrata (#167)

Marin now publishes COVID data in an actual data portal with a real API and changed the dashboard to use Tableau. That broke our scraper (because all the charts are now built in an entirely different way). The best way to fix was to rewrite on top of the Socrata data portal API, which we should happily expect to be much more stable.

This also adds caching to the Socrata API so we can make multiple calls to the same URL without actually making multiple HTTP requests. The Marin data is arranged such that a lot of different dimensions of data are combined (unlike most other portals where they are separated), and caching lets us keep the logic straightforward without making unnecessary repeated requests.

Fixes #165.
Fixes #162 (unknowns are now included in the data!).