use new letter branding instead of dvla organisation id

new code is copied stylistically from the email branding patterns.
Instead of `service.dvla_organisation`, there's now
`service.letter_branding` and `service.letter_branding_id`. However,
unlike email branding we're not currently showing a preview of the
logo. That can come later when we work out how we want to do it.

app/models/service.py

@@ -28,6 +28,7 @@ class Service():
         'email_from',
         'id',
         'inbound_api',
+        'letter_branding',
         'letter_contact_block',
         'letter_logo_filename',
         'message_limit',
@@ -303,11 +304,15 @@ class Service():
             return email_branding_client.get_email_branding(self.email_branding_id)['email_branding']
         return None
 
+    @property
+    def letter_branding_id(self):
+        return self._dict['letter_branding']
+
     @cached_property
     def letter_branding(self):
-        return letter_branding_client.get_letter_branding().get(
-            self.dvla_organisation, '001'
-        )
+        if self.letter_branding_id:
+            return letter_branding_client.get_letter_branding(self.letter_branding_id)
+        return None
 
     @cached_property
     def organisation_name(self):