Spinners for reactive loading of cells

It would be great if the static sites generated by notebooks build could show spinners for cells that are still loading. I've seen this behaviour with observable framework sites, and it's quite useful if I want to share a report with someone, they open it and can immediately see that something's still coming. Right now, for example if I have markdown-cells interweaved by plots, like typical for a report, while I have the code cells who do the loading and wrangling somewhere below in an appendix, the text paragraphs get displayed, and there is no way to tell that there are objects between markdown paragraphs until they've fully loaded.

Start at the notebooks build entry point and trace how static sites render cells and their loading state. Confirm how reactive loading is represented before deciding where the spinner belongs. Done means an opened report visibly indicates cells that are still loading, including when markdown appears before plots or other loaded objects.