Update earthquake markers based on zoom level

The earthquake markers need to be easier to click/touch when the user zooms in on an event.

The styles for the old interface are recorded in the css: https://github.com/usgs/earthquake-latest-earthquakes/blob/4ca3187edd2d3c515164c9dee386a1e5e55f2088/src/htdocs/css/map/_EarthquakeLayer.scss#L43

The marker sizes are psuedo-dynamically being read into the legend.component. The legend will need to be updated when the map is zoomed in/out.