create models for SendAirtime and AirtimeApplication

**SendAirtime**

- MSISDN (INT)
- product_key (CHAR)
- amount (INT)
- app_idd(FK)
- sent (BOOLEAN)
- created_at (DATETIME)

**AirtimeApplication**
- name (CHAR)
- ratio (INT)
- per_day (INT)
- amount (INT)
- active (BOOLEAN)
- product_key (CHAR)

**Tasks**

- Add celery beat worker that takes airtime from SendAirtime and sends to gopher.