WIP: Wrap code blocks in <iframe>s

So we can encapsulate styles/scripts and make them responsive.

To do:

- [x] Use the [react-marked](https://github.com/LinuxBasic/react-marked) fork of marked
- [x] Wrap code blocks in react-frame-component
- [x] Pass code styles to Frame component
- [ ] Pass code scripts to Frame component (optional)
- [ ] Resize iframes to content (maybe use https://github.com/davidjbradshaw/iframe-resizer)
- [ ] Define more clearly which code blocks get converted to iframes (currently all except `code`)
- [ ] Refine the `Catalog` API in regards to styles and scripts which get injected into iframes (e.g. would we want to define those per-page?)