Handle AsciiDoc better, reuse HTML pipeline filters (fixes #9263)
Created by: jirutka
This PR basically makes AsciiDoc a “first-class citizen” in GitLab, so we can reuse some HTML pipeline filters etc. The main reason why I’ve done this is to use RelativeLinkFilter (implemented in #9276) for AsciiDoc, and so fix #9263 (closed).
Since we don’t need github-markup for AsciiDoc anymore, I’ve updated the Asciidoctor gem to the latest version. The current used version (0.1.4) is ancient.
/cc @tsigo @randx @mojavelinux