Use the original metadata author in the HTML author meta tag, the PDF cover and the document index

Some templates display the effective metadata author as the author of the last change, while the page header, the history and the diff display the original metadata author:

• meta.vm (the HTML author meta tag)
• pdfcover.vm (the PDF export cover)
• getdocuments.vm (the document index)

They should display the original metadata author, so that the same author is shown everywhere for the same change (e.g. after a comment or a tag was added).

Context: phase 0 of the System user proposal (design: https://design.xwiki.org/xwiki/bin/view/Proposal/SystemUser). It's independent from it and can be done first.