Remove usa_contact_id

This removes the usa_contact_id field from the Agency model. We don't need to store this in the database, only the YAML files. 

I don't know that there's a good way to test the removal of the field. All tests pass after removal. 

Because I felt like writing a test, I've added a test that saves the agency model and tests some of the custom bits of that. This is somewhat unrelated to the change made. 
Will this fail on reverting the migration? Seems like the `RemoveField` migration doesn't have enough information to add the field back again.