Add metadata and offline storage to pass PWA check

[slightlyoff]

Also uses a smaller geographic query for sensors in order to minimize startup time, both online and from localstorage.

Small nits fixed:

- adds a <meta name="theme-color"> for full-bleed color
- sets manifest `start_url` w/ PWA query params
- adds name/short_name values for homescreen shortcut
- caches all non-PA assets, including analytics
- adds MIT license to package.json

To update the site in future, ensure that new assets are added to the list in sw.js and bump the "version" number in that file.

TODO: more aggressive cache busting for the SW impl.

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[slightlyoff]

Screenshots for the Android flow:

[slightlyoff]

And it's a desktop app too, because PWAs.