Update item.js for display of case law title

This is a request from several Canadian and European lawyers using Zotero.

+ Common law citation:
In case of "neutral reference" (now commonly used as first reference for case law in Canada and EU), there is no report... Zotero should then display the court official code.

+ Civil law citation:
Zotero should display shortTitle; because, actually, cases are also known by name in civil law jurisdictions.