add population and feature code to improve search results

In applications where users search for a city, I think it would be useful to have feature_code and population available to improve search results.

Example: Search for "Berlin". Without population and feature_code, this will turn up

a.) every district of Berlin, Germany
b.) several very small towns in other countries named "Berlin"

With feature_code and population present, one can
1.) exclude (or even delete) results with feature code "PPLX" (because that's typically not what a user would expect)
2.) order results by population, so the larger cities will be displayed first

Note 1: I also removed CityForm from admin because it hides a lot of fields (which I find confusing) - feel free to ignore that if you don't like it
Note 2: you might want to double-check the migration in this patch (or provide your own version), since it somehow meddles with the AutoSlugField (frankly I don't know why, as I didn't change that in the model).