Prepare to replace "billing_units" in usage APIs

There is no such thing as a "billing unit". The data this field
contained was also a confusing mixture of two types:

- For emails and letters, it was just "notifications_sent".

- For SMS, it was the "chargeable_units" (billable * multiplier).

This replaces the single, ambiguous "billing_units" field with
"chargeable_units" and "notifications_sent" in both usage APIs.
Once Admin is using them we can remove the old field.

app/billing/billing_schemas.py

@@ -17,7 +17,10 @@ def serialize_ft_billing_remove_emails(rows):
         {
             "month": (datetime.strftime(row.month, "%B")),
             "notification_type": row.notification_type,
+            # TEMPORARY: while we migrate away from "billing_units"
             "billing_units": row.billable_units,
+            "chargeable_units": row.chargeable_units,
+            "notifications_sent": row.notifications_sent,
             "rate": float(row.rate),
             "postage": row.postage,
         }
@@ -30,7 +33,10 @@ def serialize_ft_billing_yearly_totals(rows):
     return [
         {
             "notification_type": row.notification_type,
+            # TEMPORARY: while we migrate away from "billing_units"
             "billing_units": row.billable_units,
+            "chargeable_units": row.chargeable_units,
+            "notifications_sent": row.notifications_sent,
             "rate": float(row.rate),
             "letter_total": float(row.billable_units * row.rate) if row.notification_type == 'letter' else 0,
         }