Replace full page previews with images

[evwilkin]

Revert to images for previews, addressing both #1557 and #1562

[redallen]

I think we can create a post-build step where we start serve the built site (minus the fullscreen image thumbnails), grab the screenshots from localhost:9000/whatever (there's already a list of fullscreen pages in .cache/fullscreenPages.json that's used for the a11y tool), and then save them back to the public folder under static/some-sha-known-at-compile-time/Button/basic.png.

We would add this step to the theme.

[redallen]

Closing in favor of #1660