Use new guest list API URLs

The API now[1] accepts requests on `…/guest-list` as well as
`…/whitelist`. This commit starts using the former, which means:
- the use of ‘whitelist’ is fully gone from the admin app
- the API can stop using it, at least in URLs

1. As of https://github.com/alphagov/notifications-api/pull/2928

app/notify_client/service_api_client.py

@@ -327,11 +327,11 @@ class ServiceAPIClient(NotifyAdminAPIClient):
         return self.get(url='/service/{}/notifications/monthly?year={}'.format(service_id, year))
 
     def get_guest_list(self, service_id):
-        return self.get(url='/service/{}/whitelist'.format(service_id))
+        return self.get(url='/service/{}/guest-list'.format(service_id))
 
     @cache.delete('service-{service_id}')
     def update_guest_list(self, service_id, data):
-        return self.put(url='/service/{}/whitelist'.format(service_id), data=data)
+        return self.put(url='/service/{}/guest-list'.format(service_id), data=data)
 
     def get_inbound_sms(self, service_id, user_number=''):
         # POST prevents the user phone number leaking into our logs