Allow targeting the PDF in another document using document name and filename

Currently the PDF viewer allow only viewing PDF attached to the current document, else you need to craft a full absolute URL, which is a pain.

This improvement will provide the following syntax:

{{pdfviewer document="space.name" file="paper.pdf" /}}

The existing behavior is kept, without document defined, the current document is used, and URL starting with http:// or https:// are also supported.