upgrade to blackfriday/v2 #8502
Description
russross/blackfriday made breaking api changes and has recently released v2.0.1. This change dramatically simplifies the blackfriday.Renderer and blackfriday.Markdown interfaces, and will require some work to bring modules/markup/markdown/markdown.go up to speed. It does seem active development and support is moving to the v2 branch, so it would be good to finish off the migration sooner over later. Otherwise there has been some symbol renaming that will need to be fixed up.
