Add management command for creating Chicago border

The clear way to do this would be to take a union of the community
area shapes and get their boundares.  This is the attempt I took
with the ``export_chicago_geojson`` management command.  This
didn't work.

Instead, I'll extract it from the Census Places shapefile using
the ``chicago_geojson_from_shp`` management command.

Addresses https://github.com/sc3/cook-convictions/issues/66