Add platform admin setting for letter branding

Does two main things:
- defines what ‘brands’ we support, in terms of the ID that DVLA use
- adds a form to choose which branding a service uses (currently
  platform admin only, like email branding)

By doing this we will be able to (with some more work) preview and send
letters with a variety of different branding.

Story: https://www.pivotaltracker.com/story/show/143506905

app/notify_client/service_api_client.py

@@ -91,7 +91,8 @@ class ServiceAPIClient(NotifyAdminAPIClient):
             'created_by',
             'branding',
             'organisation',
-            'letter_contact_block'
+            'letter_contact_block',
+            'dvla_org_id',
         }
         if disallowed_attributes:
             raise TypeError('Not allowed to update service attributes: {}'.format(