MDL-37162: Use CSS to make mod show/hide icons not look "clickable" in hidden sections

Previously, the resource toolbox initialiser was using JS to find the show/hide icon, then find its section ancestor, then check if the section was hidden - only to pointlessly set pointer:auto if it was.  That didn't meet the aim stated in the comments ("Disable" show/hide icons (change cursor to not look clickable) if section is hidden), but this patch meets that goal with a single CSS rule.