Loading animation for snapshot image

[ghengeveld]

From AP-3661

Sometimes when I click on a story, the snapshot image takes a few moments to load. In the meantime, the panel where it goes is just empty. I would like to see a loader appear there immediately to maintain Storybook's snappy local feel.

[MichaelArestad]

This was overlooked in the initial implementation, but it should look like this:

Figma source

[MichaelArestad]

I bumped up priority here because sometimes they take a while to load with no indication that anything is loading.