Provide inline colour wheel widgets for theme customisation

Aid theme customisation by providing inline colour wheels in the CodeMirror documents, such as those provided by: https://www.npmjs.com/package/codemirror-colorpicker

> Regarding the customisable editor configuration, would it be in any way possible to add a colour wheel so people can click on that, or use an eyedropper-style tool, rather than having to mess about with hex codes and the like? It’d save a lot of time and fiddlyness and be nicer for people who are used to that system from other apps.

\- @Fiogan

Needs some investigation as to whether that module would be sufficient, or whether we'd be better off with a simpler hand crafted solution.