Crop editor: interpret 'cs' as centre-sliced crop.

At some point, I started using 'cs' instead of 'cc'. Let's just support both.

public/js/crop_editor.js

@@ -100,7 +100,7 @@ class CropEditor {
 		let sourceAspectRatio = source.naturalWidth / source.naturalHeight;
 
 		// Cropping from the centre?
-		if (cropMethod === "c") {
+		if (cropMethod === "c" || cropMethod === "s") {
 			// Crop vertically from the centre, using the entire width.
 			if (sourceAspectRatio <= cropAspectRatio) {
 				this.crop_width.value = source.naturalWidth;