create ability for district admin to register on the system via USSD

The purpose of this is to create the district admin user django models and api endpoint that will receive data posted from vumi go.

Includes #85 (which was meant to allow rts staff who don't have the district admin to be able to send an sms).

### User story
so that I can create learner performance data as a district admin I can register on the system

### Details
In order to for the district admin to register.
> Need to create a DistrictAdminProfile model.
    - date_of_birth
    - id_number
    -  first_name
    - last_name
    - created_at
    - district
> Expose the DistrictAdminProfile model to API endpoint