Add usage to the dashboard

Takes the number of emails and SMS fragments sent from:
https://github.com/alphagov/notifications-api/pull/273

Using these numbers it’s possible to show:
- how much of your allowance is left
- or how much you have spent

For now the allowance and rates are hard coded.

Only for users that have manage service.

tests/conftest.py

@@ -898,6 +898,19 @@ def mock_get_template_statistics(mocker, service_one, fake_uuid):
         'app.template_statistics_client.get_template_statistics_for_service', side_effect=_get_stats)
 
 
+@pytest.fixture(scope='function')
+def mock_get_usage(mocker, service_one, fake_uuid):
+
+    def _get_usage(service_id):
+        return {'data': {
+          "sms_count": 123,
+          "email_count": 456
+        }}
+
+    return mocker.patch(
+        'app.service_api_client.get_service_usage', side_effect=_get_usage)
+
+
 @pytest.fixture(scope='function')
 def mock_events(mocker):