Introduce $xwiki.getURL() for EntityReference

Use case: display a breadcrumb representing the parents of a nested document.

Currently, $xwiki.getURL() actually takes DocumentReference only. We should expend it to all EntityReference.

The proper way to do it is to implement BinEntityResourceReferenceSerializer and WikiEntityResourceReferenceSerializer in the URL module. According to vmassol, it requires too much work for 7.2.

But we can introduce getURL(EntityReference) in the XWiki class easily in the meantime.