Allow cropping when specifying thumbnailed images.
Currently, if one wants to show only a detail of a photograph, say, isolating
the head of a person, one has to download the photo, crop it and reupload it.
T67284: Insert cropped image from Scan
T39732: Wikimedia Commons needs a built-in raster image editor / photo editor
T37756: Image parameter for thumbs that scale to an exact box, cropping as necessary
T159640: Request for functionality: server-side cropping of images for commons