social media preview card image should use top portion of the image

[rlskoeser]

Right now we're only adjusting card images by width; to improve display on twitter it would be better to crop the image to show the top portion.

Here's an example showing the problem: https://twitter.com/ShakesCoAt100/status/1473292180600942593

The image preview on twitter looks blank, but in fact there is writing at the top of the card.

Adding a region & size something like this pct:0,0,99,40/1200, to the iiif image we're using for social previews will probably work for the majority of cards (the ones that are standard size); but it may not work well for images that are partial cards. To generate a proper image url that takes original image size into account, we'd need to write a custom view that would request the image info and generate the region size accordingly, which may be more effort than is warranted at this point.