This starts using the sms_{cost, charged, free_allowance_used}
fields in the new API to replace the "get_free_paid_breakdown"
function we had before, which could not support multiple rates.
In order to use "get_free_paid_breakdown" the calling method had
to store a "cumulative" variable to calculate the free allowance
used so far, which is now done by the API.
To calculate the data for conftest.py, I had to start from the
bottom ("April") and manually calculate the free allowance used
to emulate the API - this is what "cumulative" used to do.
d798a0d60f