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.
And why all the fields below are only set if ``title_arXiv`` is in metadata? Is it always there?

It seems a bit strange to me that all the code below only gets executed if ``title_arXiv`` is there.