Use new "cost" field in usage APIs

The previous, manual calculation could be incorrect depending on
which SMS rates the free allowance was attributed to.

The new field also supersedes the old "letter_total" bolt-on so we
can get cost information consistently for both types.

app/templates/views/usage.html

@@ -59,7 +59,7 @@
         <div class='govuk-grid-column-one-third'>
           <div class="keyline-block">
             {{ big_number(
-              (sms_chargeable * sms_rate),
+              sms_cost,
               'spent',
               currency="£",
               smaller=True