Map performance improvements

[almccon]

We can improve the map performance in a few ways:

1. reduce the amount of data by filtering according to the viewport (see Change data loading behavior #18). This only helps when we're zoomed in on the map. If you are viewing the whole city, you still need to load all the buildings.

2. Use clustering (via the Leaflet plug-in, presumably) to reduce the number of features we render on the map, even though we still would load all the buildings from the server.

3. Switch the map library to a GL map (Tangram or MapboxGL). Either of those should handle 20K points just fine. My main concern with that approach would be to check what browsers we need to support, and how well they handle GL.