Uses geojson tiles for the "minority" layer

The python side is simplified, offering only one entry point to retrieve census tracts.

The Javascript side is also simplified, though it wouldn't appear that way on first glance. The flow used to be: 

1. load the first page (~200) census tracts
2. keep loading pages after the first completes
3. in parallel to the additional loads, start loading statistic information
4. after getting the response per statistic, if both geo and statistic are present, draw a shape

With geojson tiles, we let the tile layer do the work of loading census tracts. We still need to know to start loading statistic info, though, I've added a hook to the tile layer which fires whenever a tile loads. This hook effectively performs steps 3 and 4 in the previous flow.

This also removes the geo information from dataStore, as it's now handled by the tile layer.