Make Redcarpet Markdown renderer thread-safe

Merged Stan Hu requested to merge sh-thread-safe-markdown into master

The Redcarpet library is not thread-safe as described in https://github.com/vmg/redcarpet/issues/570. Since we instantiate the Redcarpet renderer in a class variable, multiple Sidekiq threads can access the work buffer and corrupt the state. We work around this issue by memoizing the renderer on a thread basis.

Closes #36637 (closed)