Style code snippet block within Wagtail editor

[dcgauld]

It'd be nice if we could style the author-facing code snippet block a little more appropriately within Wagtail, using a monospaced font and (maybe) syntax highlighting.

Currently it looks like this:

We might be able to use an existing Wagtail feature for this, or inject custom styles. Additionally, it'd be nice if we could disable spellchecking, autocapitalisation, etc too.