Pages grid: Improve the preview field

[jameskoster]

When a site doesn't use featured images for pages (not uncommon) the grid view is lacklustre:

The gray thumbnails look a bit broken, or like a loading state that never resolves. How could this be improved?

One option to try is loading the content preview, similar to the templates grid.

A more long-term solution could be to allow the user to decide via in the view options menu:

cc @ellatrix @richtabor

[jasmussen]

Content preview sounds like a compelling feature to lift over from the Patterns section. But there are some performance considerations especially for very long pages. Since it already exists for patterns, I wonder: can we apply it as a fallback to the Featured Image to start? That way you'd always avoid the gray blotch. We might be able to add the drop-down option later on, should you want to see only that.

[jameskoster]

I'm not entirely convinced about the fallback, in most cases a content preview is probably more useful so it seems a bit strange to prioritise the featured image. It may also be confusing why different pages have different preview types.

It would be good to get some technical feedback on the performance side of things. I'm not sure that rendering page previews would be all that different to template previews? So if we're doing it in one (two) places already, why not here as well?

The gray blotch could also be improved by clarifying that it relates to an image. We might use the gray diagonal stroke like the Image block placeholder state.