[web] Render markdown (or at least a meaningful subset of it) in comments

[whisperity]

In comments, the most major Markdown-ish features should be rendered in the web interface: bold, italic, underline, and monospace highlight with backticks. (Links and images are not really needed.) There are definitely some NodeJS libs that can render Markdown to inline HTML.

[vodorok]

This is a pretty popular lib for it, we should deffo try it: https://www.npmjs.com/package/markdown-it