Fixed the scrolling/list close bug

At least in Chrome, when you scroll to the top/bottom of the timepicker list, if you keep going, the whole body will start to scroll. It's very easy to scroll too far that the cursor ends up not hovering over the list anymore, which hides the list.

This commit sets overflow:hidden on <html> and <body> whenever the list is visible via a new .js-timepicker-noscroll class that's set on <html>, effectively preventing this bug.