Prepare to switch to "chargeable_units" in API

This is so we can migrate from "billing_units" to this new field in
the Admin app, without breaking anything in between.

tests/app/billing/test_rest.py

@@ -240,15 +240,21 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     )
 
     assert len(json_response) == 3
+
     assert json_response[0]['notification_type'] == 'email'
     assert json_response[0]['billing_units'] == 275
+    assert json_response[0]['chargeable_units'] == 275
     assert json_response[0]['rate'] == 0
     assert json_response[0]['letter_total'] == 0
+
     assert json_response[1]['notification_type'] == 'letter'
     assert json_response[1]['billing_units'] == 275
+    assert json_response[1]['chargeable_units'] == 275
     assert json_response[1]['rate'] == 0.33
     assert json_response[1]['letter_total'] == 90.75
+
     assert json_response[2]['notification_type'] == 'sms'
     assert json_response[2]['billing_units'] == 825
+    assert json_response[2]['chargeable_units'] == 825
     assert json_response[2]['rate'] == 0.0162
     assert json_response[2]['letter_total'] == 0

tests/app/dao/test_ft_billing_dao.py

@@ -508,21 +508,25 @@ 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 == 365
     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')
 
 
@@ -534,21 +538,25 @@ 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 == 1
     assert results[1].billable_units == 1
+    assert results[1].chargeable_units == 1
     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')