Display the original metadata author in the "last author" livetable and Live Data column

The page header, the history, the diff and the search display the original metadata author of a page (the user who triggered the last change), but the "last author" column of livetables and Live Data (doc.author) displays the effective metadata author (the author whose rights apply).

The two differ whenever a change only updates the original metadata author, e.g. adding a comment or a tag: the page header then says "Last modified by Alice" while the "last author" column of the same page still shows the previous editor.

Proposal

• Make the doc.author column display, filter and sort on the original metadata author, in the central livetable results / Live Data livetable source implementation, so that all applications using that column (AppWithinMinutes, Menu, Help, XClass, Index, …) follow without changing their column definitions.
• HQL queries in code and the REST API stay unchanged.

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.