Implement low credit alerts in billing

Having the billing API alert the account holder when there credits drop below a specified threshold.

Currently the billing API marks this is marked as TODO at https://github.com/praekelt/vumi-go/blob/e4320d12c81bc122cc9b70f96492c59ee1a58290/go/billing/api.py#L755-L757
I'm not sure that this is equivalent, ie. 
```
>>> credit_balance = 97
>>> alert_credit_balance = 100
>>> credit_amount = 5
>>> credit_balance < alert_credit_balance and credit_balance + credit_amount > alert_credit_balance
True
>>> credit_balance <= alert_credit_balance < credit_balance + credit_amount
True
>>> credit_balance - credit_amount <= alert_credit_balance < credit_amount
False
```