Introduce hotspot layer

[DarrinWeber]

As cities reach a 7-day threshold, they should be spotlighted.

[mayafrederick]

@DarrinWeber I changed the is_hotspot variable to be logical. What I have pushed does not incorporate the hotspot circling into the leaflet, I can't figure out how to do that. I have commented out (line 441-455) my most recent attempt. I think it has something to do with the fact that I don't have latitude and longitude points but I am not sure.