Fix incorrect chargeable_units column

This was added to migrate away from the vague "billing_units" field,
but without fixing the inconsistent data behind it:

- For emails and letters, "billing_units" was just the number sent.

- For SMS, "billing_units" really was the chargeable_units.

To avoid confusion we need two fields to represent the original mix
of data - this exposes "notifications_sent" in both APIs.

tests/app/billing/test_rest.py

@@ -178,6 +178,7 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert letter_row["cost"] == 9.9
     assert letter_row["free_chargeable_units"] == 0
     assert letter_row["charged_units"] == 30
+    assert letter_row["notifications_sent"] == 30
 
     assert sms_row["month"] == "April"
     assert sms_row["notification_type"] == "sms"
@@ -189,6 +190,7 @@ def test_get_yearly_usage_by_monthly_from_ft_billing(admin_request, notify_db_se
     assert sms_row["cost"] == 4.212
     assert sms_row["free_chargeable_units"] == 4
     assert sms_row["charged_units"] == 26
+    assert sms_row["notifications_sent"] == 30
 
 
 def set_up_yearly_data():
@@ -263,6 +265,7 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[0]['cost'] == 0
     assert json_response[0]['free_chargeable_units'] == 0
     assert json_response[0]['charged_units'] == 275
+    assert json_response[0]['notifications_sent'] == 275
 
     assert json_response[1]['notification_type'] == 'letter'
     assert json_response[1]['billing_units'] == 275
@@ -272,6 +275,7 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[1]['cost'] == 90.75
     assert json_response[1]['free_chargeable_units'] == 0
     assert json_response[1]['charged_units'] == 275
+    assert json_response[1]['notifications_sent'] == 275
 
     assert json_response[2]['notification_type'] == 'sms'
     assert json_response[2]['billing_units'] == 825
@@ -281,3 +285,4 @@ def test_get_yearly_billing_usage_summary_from_ft_billing(admin_request, notify_
     assert json_response[2]['cost'] == 13.3002
     assert json_response[2]['free_chargeable_units'] == 4
     assert json_response[2]['charged_units'] == 821
+    assert json_response[2]['notifications_sent'] == 825

tests/app/dao/test_ft_billing_dao.py

@@ -489,7 +489,7 @@ def test_fetch_monthly_billing_for_year_variable_rates(notify_db_session):
     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].chargeable_units == 2
     assert results[1].rate == Decimal('0.36')
     assert results[1].cost == Decimal('0.36')
     assert results[1].free_chargeable_units == 0
@@ -614,7 +614,7 @@ def test_fetch_billing_totals_for_year_variable_rates(notify_db_session):
     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].chargeable_units == 2
     assert results[1].rate == Decimal('0.36')
     assert results[1].cost == Decimal('0.36')
     assert results[1].free_chargeable_units == 0