Service deafault radio checked by default, existing tests pass.

app/main/forms.py

@@ -452,10 +452,10 @@ class LetterTemplateForm(EmailTemplateForm):
         choices=[
             ('first', 'First class'),
             ('second', 'Second class'),
-            ('service_default', "Service default"),
+            ('None', "Service default"),
         ],
         validators=[DataRequired()],
-        default='service_default'
+        default='None'
     )
 
     subject = TextAreaField(

app/notify_client/service_api_client.py

@@ -178,7 +178,7 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             data.update({
                 'postage': postage
             })
-        elif postage == "service_default":
+        elif postage == 'None':
             data.update({
                 'postage': None
             })

tests/app/main/views/test_templates.py

@@ -1091,7 +1091,7 @@ def test_should_show_interstitial_when_making_breaking_change(
     }
 
     if template_type == "letter":
-        data.update({"postage": "service_default"})
+        data["postage"] = 'None'
 
     response = logged_in_client.post(
         url_for('.edit_service_template', service_id=service_id, template_id=template_id),