Add additional final fields to Disposition, Conviction

Add the following fields to the Disposition and Conviction models:

* final_chrgdesc
* final_chrgtype
* final_chrgclass

These make Conviction queries easier, especially because statute ids
can be hard to interpret and might not map to IUCR codes.

The fields trickle up from RawDisposition and can be loaded with this
snippet:

from django.db import transaction
from convictions_data.models import Disposition

with transaction.atomic():
    for d in Disposition.objects.select_related('raw_disposition').all():
        d.load_field_from_raw('chrgdesc')
        d.load_field_from_raw('chrgtype')
        d.load_field_from_raw('chrgclass')
        d.load_field_from_raw('ammndchrgdescr')
        d.load_field_from_raw('ammndchrgtype')
        d.load_field_from_raw('ammndchrgclass')
        d.save()