Markdown component

[xanthipe]

This needs to take Markdown-formatted text and convert it back into displayed HTML.

The Markdown renderer needs to be able to handle everything we currently use (including complex tables), and the intention is to use this in a CMS-like way later, so we might want to consider integrating a CMS package into the code sooner than later.

https://github.com/remarkjs/react-markdown is potentially a good place to start.