Added baseline hashing capabilities to CountyInmate model and scrape_inmates command

### What this change doesn't do
This change does NOT re-work our models at all, except to add a new field associated with an inmate, called `person_id`. It's a bit scaled back from the [earlier pull](https://github.com/sc3/cookcountyjail/pull/117).

### What it does do
- In addition to the new `person_id` field on our model, the data to fill this field is now created at scrape-time, with a SHA-256 hashing function, from four data: 
    - name
    - birthdate
    - (first letter of) race
    - gender
- The south migration for the new schema is also done.
- This should at least allow us a certain amount of backwards compatibility between API 1.0 and whatever comes after. If we ever want to try to port the data we scrape after this pull, we will have the hashed `person_id` (which we otherwise can't recreate after the person leaves the Cook County Sheriff's website).