extra arXiv mapping

Add extra fields from arxiv (same as in oaiharvest) to the form fields (see [asana task](https://app.asana.com/0/8772567966361/12984151070479)).

arXiv journal names mapping depends on https://github.com/jmartinm/inspire-next/commits/overlay-changes.
Ideally after import the Subjects dropdown in the form would be prefilled based on the categories from arXiv. In that way the generated MarcXML would contain the original arxiv subjects and the translated ones with ```$$2INSPIRE```