[CODE] Add "Location (County)" Filter

Background: please see #58 for a roadmap of the Additional Filters request.

We currently have a basic filtering system for filtering through prosecutors by role.

This feature request is for implementing an additional filter, which filters based upon a state's county.

In order for this to work, Filter Location by State (#109) must be complete. You can tackle that one first, or both these issues at the same time.

Ideally, this would be rendered a drop-down or an auto-populated text input. If you would like to take on this issue, please leave a comment.