Allow a configurable custom crop for showcased images

[seanaery]

As encountered with the Rush Letters collection, it'd be helpful to be able to indicate a custom crop for items selected as showcase images on a collection portal. E.g., show the part of a letter where George Washington signed.

By default, the current double-showcase area renders images this way, using a combination of image server and CSS3 background properties:

• Crop off the outer 5% of the image (fairly predictably removes the platen)
• Render the remaining image at 390px wide as a background image centered vertically & horizontally
• Scale up if needed to reach 500px tall (height of the showcase box)