Data views: Add loading indicator for preview field

[jameskoster]

Related: #59792

Here's Table layout:

preview.mp4

It feels very clunky how the templates load piecemeal. It's particularly bad in this layout due to the reflow that occurs when the templates load.

Here's Grid layout:

grid.mp4

The fixed height helps a bit by eliminating the reflow, but it still feels very sluggish.

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To address we can add a fixed height to the template previews in table layout, and add a loading indicator which obscures the preview until fully loaded.

[t-hamano]

I tried solving it once in #61408, but it wasn't the ideal approach. let me remove myself from assignees as I can't think of any other solution at the moment.