This fixes a few accessibility problems with drag and drop

LMS-1940
LMS-1904

There's not a lot we can do to make a drag and drop interface accessible, if it relies on the user's ability to see something particular about the drop target. I've added the appropriate ARIA properties to the draggables and drop target and enabled some keyboard navigation, but it isn't enough. You're supposed to allow the user to move draggables around the screen with the keyboard; I'm not sure if it's worth putting in that effort if the user still needs to be able to see where to drag the element.

@talbs what do you think?