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/models/service.py

@@ -40,6 +40,7 @@ class Service():
         'volume_sms',
         'volume_letter',
         'consent_to_research',
+        'count_as_live',
     }
 
     TEMPLATE_TYPES = (