converter: run conversion asynchronously via Celery if offline=True

[GraemeWatt]

The convert_and_store function calls download_submission with an argument offline=True, in turn calling convert_zip_archive which performs the conversion using hepdata-converter-ws-client, hepdata-converter-ws and hepdata-converter. However, the conversion is performed synchronously and large submissions can encounter a 5-minute server timeout. It would be better if the offline=True option was propagated to hepdata-converter-ws which would then run hepdata_converter.convert asynchronously as a Celery task, writing the output directly to the converted folder of the data directory.