slow map rendering with a lot of map markers

[saylorsd]

When a lot of markers are on the map, it updates very slowly.

I think we may have too many markers to use the react-map-gl <Marker/> component.

The one solution I can think of is to render the markers server-side (on mapbox). This would involve reflecting the state of our app in a Mapbox style object and passing it to the Map component's mapStyle prop.

[qualitymanifest]

I've got a reasonably fast version of this going with a geoJSON Layer instead of Markers. Needs a little work but I'll make a PR tomorrow.