Dataset Page: Should lazy load thumbnails to better support slower storage media and improve page responsiveness.

[kcondon]

The dataset page needs to be more responsive when using slower storage media such as s3 and a number of files have thumbnails (pdfs, images).

One way that is used elsewhere in search results is to load the card with a default icon so the page can load and then load the thumbnail as it becomes available.

Otherwise page load is slow as is scrolling through pages of files.

My Data could potentially benefit from this but it is currently only displaying icons rather than thumbnails.

[landreev]

OK, looks like I was able to get it to work with less code than I was going to write originally.
Made the PR: #4135
moving into Code Review.