Update service schema to add letter_logo_filename

Added the filename of a service's letter logo to the service schema. We want
this in the schema so that it is possible to call
`current_service.letter_logo_filename` from notifications-admin and to pass this value
through to template-preview.

tests/app/service/test_rest.py

@@ -139,6 +139,7 @@ def test_get_service_by_id(admin_request, sample_service):
     assert json_resp['data']['dvla_organisation'] == '001'
     assert json_resp['data']['prefix_sms'] is True
     assert json_resp['data']['postage'] == 'second'
+    assert json_resp['data']['letter_logo_filename'] == 'hm-government'
 
 
 @pytest.mark.parametrize('detailed', [True, False])