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/main/forms.py

@@ -933,9 +933,13 @@ class OnOffField(RadioField):
 
 class ServiceOnOffSettingForm(StripWhitespaceForm):
 
-    def __init__(self, name, *args, **kwargs):
+    def __init__(self, name, *args, truthy='On', falsey='Off', **kwargs):
         super().__init__(*args, **kwargs)
         self.enabled.label.text = name
+        self.enabled.choices = [
+            (True, truthy),
+            (False, falsey),
+        ]
 
     enabled = OnOffField('Choices')