Add map of suburbs

* Add GeoJSON file for suburban census places
* Add a new slide, ``suburbs-map`` to contain the map
* Refactor map classes to make them more reusable
* Add a new map class and initialization function for the suburbs
  map
* Adjust values and bins on the new map and the community area map
  to use convictions per 1000 instead of convictions per capita
* Remove trailing whitespace

Addresses https://github.com/sc3/cook-convictions/issues/77