Add a clearer key to the map

[pelmers]

Basically implement the TODO at

metro-lens/src/client/index/MapComponent.tsx

Line 750 in 42f132b

{/* TODO add a map legend, e.g. https://docs.mapbox.com/mapbox-gl-js/example/updating-choropleth/ */}

, not sure on design yet. Ideas welcome!

Maybe the color key could also let you change the color scheme, to make it prettier or to help with distinguishing red and green.