Replace manual SMS monthly calculations with API

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.

app/templates/views/usage.html

@@ -99,7 +99,7 @@
             {% endcall %}
             {% call field(align='left') %}
               {{ big_number(
-                (item.sms_rate * item.sms_charged) + item.letter_cost,
+                item.sms_cost + item.letter_cost,
                 currency="£",
                 smallest=True
               ) }}