Allow excluding services from live services count

Adds a front end for:
https://github.com/alphagov/notifications-api/pull/2417

> Sometimes we have to make a few services for what really is one
> service, for example GOV.UK Pay and GOV.UK Pay Direct Debit. We also
> have our own test services which aren’t included in the count of live
> services. We currently count these as one service by not including
> them in the beta partners spreadsheet.

app/notify_client/service_api_client.py

@@ -88,6 +88,7 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             'volume_sms',
             'volume_letter',
             'consent_to_research',
+            'count_as_live',
         }
         if disallowed_attributes:
             raise TypeError('Not allowed to update service attributes: {}'.format(