Use Preview Versions Of Images For Media Library

[Undistraction]

Is your feature request related to a problem? Please describe.
Currently the default media library uses the full sized uploaded images in its previews. We have a large number of large (2600px wide) images, and once there are more than 20 or so in the library, it becomes sluggish and unresponsive. So even though each image is only displayed at 276px wide, the image is actually the original - nearly ten times wider than it needs to be.

Describe the solution you'd like
When uploading an image a preview version of the image should be saved, and the preview, not the uploaded image used in image previews.

Describe alternatives you've considered
There is no available alternative as far as I can see as the upload and previewing are handled by the library.

Note that I'm using Gitlab Self-hosted as the backend, so it may be specific to how Netlify CMS handles images there.

[tomrutgers]

Duplicate of #946