Pass through contact block from service to letter

Whatever a user has entered for their service’s contact block should
appear in the right place in the file we give to DVLA.

The work to output in the right fields in the DVLA file has already been
done. We just weren’t passing it through. This commit passes it through.

tests/app/celery/test_tasks.py

@@ -994,6 +994,7 @@ def test_build_dvla_file(sample_letter_template, mocker):
 
     # Named arguments
     assert mocked_letter_template.call_args[1]['numeric_id'] == 999
+    assert mocked_letter_template.call_args[1]['contact_block'] == 'London,\nSW1A 1AA'
 
 
 def test_build_dvla_file_retries_if_all_notifications_are_not_created(sample_letter_template, mocker):