Expand RIS fields written to download

At the moment the download is a bare-bones set of fields (type, year, author, title, journal, publisher, volume, issue, start page, end page, doi) so any additional fields in the input are lost, and extra fields from the Open Alex API aren't built in (e.g. URLs).

Need to ensure upload fields are merged with the output before export, and that additional Open Alex fields are passed over.