Extract handling of relative file links to its own HTML filter
Created by: jirutka
I’ve extracted rebuilding of relative links from GitlabMarkdownHelper to its own HTML filter (RelativeLinkFilter) as suggested in comment here.
My motivation for this is to fix #9263 (closed) and this is the first step. HTML filters can be easily reused for both Markdown and AsciiDoc.
However, I’m not quite sure about this, the code around Markdown rendering is very… complicated. @randx, could you please review it and give me some feedback?