Migrate some markdown-based post-processing into a separate HTML-based method

[isoos]

• Migrate markdown post-processing from markdown nodes to parsed HTML #8590
• Migrates unsafe URL filter and task list processing.
• The unsafe filter changed, as the new logic replaces the entire element with its text content, while the prior logic removed only the href/src attribute (see test for the change).
• Also added further test for the relative link rewrite (with its migration in a subsequent PR).