Fix logic for showing and hiding checkbox

The ‘make this default’ checkbox should be shown, except when:
- the user is adding their first email reply to address (because the
  first one has to be the default)
- they’re editing the existing default (because they can’t change it
  to be not default)

app/main/views/service_settings.py

@@ -474,12 +474,17 @@ def get_service_verify_reply_to_address_partials(service_id, notification_id):
     first_email_address = current_service.count_email_reply_to_addresses == 0
     notification = notification_api_client.get_notification(current_app.config["NOTIFY_SERVICE_ID"], notification_id)
     replace = request.args.get('replace', False)
+    replace = False if replace == "False" else replace
+    existing_is_default = False
+    if replace:
+        existing = current_service.get_email_reply_to_address(replace)
+        existing_is_default = existing['is_default']
     verification_status = "pending"
     is_default = True if (request.args.get('is_default', False) == "True") else False
     if notification["status"] in DELIVERED_STATUSES:
         verification_status = "success"
         if notification["to"] not in [i["email_address"] for i in current_service.email_reply_to_addresses]:
-            if replace and replace != "False":
+            if replace:
                 service_api_client.update_reply_to_email_address(
                     current_service.id, replace, email_address=notification["to"], is_default=is_default
                 )
@@ -505,6 +510,7 @@ def get_service_verify_reply_to_address_partials(service_id, notification_id):
             notification_id=notification_id,
             verification_status=verification_status,
             is_default=is_default,
+            existing_is_default=existing_is_default,
             form=form,
             first_email_address=first_email_address,
             replace=replace