Site UX Change: Code Walkthrough Changes

[alisonhall]

This ticket would have to be done separately from #109

Being able to view all of the HTML/CSS/JS of a code example at once would be useful, since it isn't immediately obvious if there is a different file/language in the example until someone selects the relevant step.

Having the code be themed would be useful to help distinguish standard keywords, and style any comments differently.

The Monaco Editor is one option for this custom code view, which allows code themes. Using the diff feature would be another way of highlighting the changes for the different steps.

This ticket should be broken up into smaller chunks:

1. Having HTML/CSS/JS all in one widget (possibly tabs, like a Codepen)
2. Syntax highlighting
3. Code Diff.