Store original filename as S3 metadata

By doing this we no longer have to store it in the session. This is the
last thing that’s currently in the session, so removing it means we can
drop session storage for file uploads entirely.

app/main/views/send.py

@@ -555,6 +555,7 @@ def _check_messages(service_id, template_id, upload_id, preview_row, letters_as_
             notification_count=len(recipients),
             template_id=str(template_id),
             valid=True,
+            original_file_name=request.args.get('original_file_name'),
         )
     else:
         session['file_uploads'].pop(upload_id)