Add `.is-locked` CSS class

It would be awesome if a CSS class was added for “lock mode” on mobile. This would allow users to add custom CSS that uses a selector for lock mode, which would be super useful!

The main use case I have for this is to disable the bullet zoom-in when in lock-mode for checkbox items so I don’t accidentally zoom-in when I just mean to check it off. It would look something like this (.is-locked would be the new CSS class):

 * Disable bullet click triggering zoom for checkbox items on
 * mobile when in lock mode.
.is-mobile .is-locked .Node.has-checkbox .Node-bullet {
    pointer-events: none;

That would actually be very usefull.

Absent this I think I would simply want to do

.is-mobile .Node.has-no-children {
pointer-events: none;