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.

tests/app/dao/test_ft_billing_dao.py

@@ -434,24 +434,28 @@ def test_fetch_monthly_billing_for_year(notify_db_session):
     assert results[0].notification_type == 'email'
     assert results[0].notifications_sent == 30
     assert results[0].billable_units == 30
+    assert results[0].chargeable_units == 0
     assert results[0].rate == Decimal('0')
 
     assert str(results[1].month) == "2016-04-01"
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 30
     assert results[1].billable_units == 30
+    assert results[1].chargeable_units == 30
     assert results[1].rate == Decimal('0.30')
 
     assert str(results[1].month) == "2016-04-01"
     assert results[2].notification_type == 'letter'
     assert results[2].notifications_sent == 30
     assert results[2].billable_units == 30
+    assert results[2].chargeable_units == 30
     assert results[2].rate == Decimal('0.33')
 
     assert str(results[3].month) == "2016-04-01"
     assert results[3].notification_type == 'sms'
     assert results[3].notifications_sent == 30
     assert results[3].billable_units == 30
+    assert results[3].chargeable_units == 30
     assert results[3].rate == Decimal('0.162')
 
     assert str(results[4].month) == "2016-05-01"
@@ -469,24 +473,28 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     assert results[0].notification_type == 'letter'
     assert results[0].notifications_sent == 1
     assert results[0].billable_units == 1
+    assert results[0].chargeable_units == 1
     assert results[0].rate == Decimal('0.33')
 
     assert str(results[1].month) == "2018-05-01"
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 2
     assert results[1].billable_units == 2
+    assert results[1].chargeable_units == 2
     assert results[1].rate == Decimal('0.36')
 
     assert str(results[2].month) == "2018-05-01"
     assert results[2].notification_type == 'sms'
     assert results[2].notifications_sent == 1
     assert results[2].billable_units == 4
+    assert results[2].chargeable_units == 4
     assert results[2].rate == Decimal('0.015')
 
     assert str(results[3].month) == "2018-05-01"
     assert results[3].notification_type == 'sms'
     assert results[3].notifications_sent == 2
     assert results[3].billable_units == 5
+    assert results[3].chargeable_units == 5
     assert results[3].rate == Decimal('0.162')
 
 
@@ -517,21 +525,25 @@ def test_fetch_billing_totals_for_year(notify_db_session):
     assert results[0].notification_type == 'email'
     assert results[0].notifications_sent == 365
     assert results[0].billable_units == 365
+    assert results[0].chargeable_units == 0
     assert results[0].rate == Decimal('0')
 
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 365
     assert results[1].billable_units == 365
+    assert results[1].chargeable_units == 365
     assert results[1].rate == Decimal('0.3')
 
     assert results[2].notification_type == 'letter'
     assert results[2].notifications_sent == 365
     assert results[2].billable_units == 365
+    assert results[2].chargeable_units == 365
     assert results[2].rate == Decimal('0.33')
 
     assert results[3].notification_type == 'sms'
     assert results[3].notifications_sent == 365
     assert results[3].billable_units == 365
+    assert results[3].chargeable_units == 365
     assert results[3].rate == Decimal('0.162')
 
 
@@ -543,21 +555,25 @@ def test_fetch_billing_totals_for_year_variable_rates(notify_db_session):
     assert results[0].notification_type == 'letter'
     assert results[0].notifications_sent == 1
     assert results[0].billable_units == 1
+    assert results[0].chargeable_units == 1
     assert results[0].rate == Decimal('0.33')
 
     assert results[1].notification_type == 'letter'
     assert results[1].notifications_sent == 2
     assert results[1].billable_units == 2
+    assert results[1].chargeable_units == 2
     assert results[1].rate == Decimal('0.36')
 
     assert results[2].notification_type == 'sms'
     assert results[2].notifications_sent == 1
     assert results[2].billable_units == 4
+    assert results[2].chargeable_units == 4
     assert results[2].rate == Decimal('0.015')
 
     assert results[3].notification_type == 'sms'
     assert results[3].notifications_sent == 2
     assert results[3].billable_units == 5
+    assert results[3].chargeable_units == 5
     assert results[3].rate == Decimal('0.162')