Create standalone Sandcastle mode

[OmarShehata]

Right now, if you click "Open in New Window" in Sandcastle it will requests from ion will fail, and you'll just see a blue sphere.

This PR creates a page to render any Sandcastle example as a standalone page that opens when you click "Open in a New Window". So any Sandcastle link that looks like:

https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/#c=ZYw7C8Iw...

Will now also be accessible at:

https://cesiumjs.org/Cesium/Build/Apps/Sandcastle/Standalone/#c=ZYw7C8Iw...

The other motivation to do this is to be able to show Sandcastle examples on mobile. You can easily share the second link now (or just click the open in new window button).

The other issue is, at least on my iPhone 5s Safari, the current open in new window blob URL just loads the HTML as text and doesn't actually render.

If there's a better way to get Sandcastle to look nice on mobile I'm happy to hear it.

I also added a few dependencies to get this to work, but I wonder if these should be added to devDependencies instead?

[cesium-concierge]

Thanks for the pull request @OmarShehata!

• ✔️ Signed CLA found.
• ❔ CHANGES.md was not updated.
  • If this change updates the public API in any way, please add a bullet point to CHANGES.md.

Reviewers, don't forget to make sure that:

• Cesium Viewer works.
• Works in 2D/CV.
• Works (or fails gracefully) in IE11.

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[mramato]

Thanks @OmarShehata. Unfortunately this PR will have to take a completely different approach in order for us to be able to merge it. Sandcastle is intentionally a client-side only app that will run in any statically hosted web-server, so we can't rely on Node.js specific routes or back-end code.

[hpinkos]

Closing this since this is not the approach we want to take here.

[emackey]

@OmarShehata Take a look at #5858, I think it's relevant.