Display county boundaries

Server-side:
* Makes sure to check the shapes' centroids (as well as their four corners) when determining whether or not they should be drawn
* Adds an HTTP end point for county geojson data

Client-side:
* Removes a layer of depth to a few of the top-level objects
* Fixes bug in which the centroid (which is used to place bubbles) field name changed
* Adds county shape as a tile layer
* County shape changes color if zoomed out enough (when census tracts no longer back it)