Queries to help analyze charge class of cases

Populate the final_statute, final_chrgdesc, final_chrgtype,
final_chrgclass and IUCR-related fields after all the other fields have
been loaded.  This fixes a bug that was caused by the field names are
returned alphabetically by Postgres and thus always setting the value to
the alphabetically first item instead of the amended version, if it
exists.

Add a management command, ``export_cases_by_class`` to export a table of
the number of cases for each felony class. This mirrors the table at
https://performance.cookcountyil.gov/Public-Safety/Number-Of-Felony-Cases-Filed-By-Felony-Class/kcfs-dufb

Add  another management command, ``export_cases_class_change`` to export
a table of how cases class change between the initial and amended
classes.

The heavy lifting of these management commands is done by queryset
methods.

Addresses https://github.com/sc3/cook-convictions/issues/144