[PoC] Embed preview of all pages in Gatsby Admin

[mxstbr]

Note: based on #25635, until that is merged you'll also see those changes here

This is a proof of concept for a "sitemap" in Admin. It renders a list of all pages of a Gatsby site, including a preview of every one! 😱 Here is what the sitemap looks like for Admin itself (meta!):

We don't necessarily have to merge this as-is, I'll leave that decision up to @shannonbux. However, this proves it's possible to show pages in Admin! 🎉

Technically, this uses the GatsbyPage provider I added to the recipes GraphQL API in #25248. I fetch that data and then render an <iframe> for every page for the "preview" 🙈 We should definitely not this for a list of all pages due to perf concerns with hundreds of iframes. However, doing it for one page at a time (or a truncated, scrollable list of pages where we only show 5-10 at a time) seems feasible!

[gatsby-cloud-staging]

Your pull request can be previewed in Gatsby Cloud: https://build-6f0bef91-196b-4ab4-ab91-9f35eb88b2d8.staging-previews.gtsb.io

[wardpeet]

Note: based on #25635, until that is merged you'll also see those changes here

You can use https://github.blog/changelog/2020-05-19-pull-request-retargeting/ & https://wchargin.github.io/posts/managing-dependent-pull-requests/ to make this flow better

[mxstbr]

@pieh pointed out in chat that we can only really render 4 pages at a time in iframes:

There is some trickiness to displaying lot of actual iframes, because you will hit connection limit issue, because of some xhr polling or event source stuff that webpack-dev-server does - check #12225 (this mentions tab, but I assume iframe situation is kind of like tabs)

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You can use https://github.blog/changelog/2020-05-19-pull-request-retargeting/ & https://wchargin.github.io/posts/managing-dependent-pull-requests/ to make this flow better

Ohhh I didn't realise GitHub added better support for this! Nice 👍

[mxstbr]

Closing for now as we're not going to ship this as-is!