add type conversion to report downloads

"sent" is fine as an internal marker but not very obvious to the end
user that it specifically refers to international messages. We now
say "Sent internationally" in the CSV

tests/app/test_model.py

@@ -111,6 +111,7 @@ def test_notification_for_csv_returns_correct_job_row_number(notify_db, notify_d
     ('email', 'permanent-failure', 'Email address doesn’t exist'),
     ('sms', 'temporary-failure', 'Phone not accepting messages right now'),
     ('sms', 'permanent-failure', 'Phone number doesn’t exist'),
+    ('sms', 'sent', 'Sent internationally'),
     ('letter', 'permanent-failure', 'Permanent failure'),
     ('letter', 'delivered', 'Delivered')
 ])