button for using grayscale

[eroux]

I just got an epiphany: for areas with very low bandwidth (which is an important part of the target audience), it would be nice to have a button (in the little viewer box) to use grayscale images, so that the images load faster (there's a iiif option to request grayscale).

Note that it shouldn't be the default because we also have a lot of binary images that would be much bigger in grayscale. If there's an easy way to detect images that are color images and have the button only appear in that case (and be the default) that's perfect, but it's probably more dev...