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.
It is not granted that what the user inputs will be found in the KB. If it is not found ```metadata['publication_info']['journal_title']``` will contain ``None``.

Instead if there is no match in the KB the value inputted by the user should be left.