a11y fix for IE 11
a11y fix for IE 11

Changes the outline-style to dotted to improve visibility.

Also, IE 11 doesn't support outline-offset, which is what we use to ensure that
the draggable item outline is visible against the background.

So we've used an inset drop-shadow for normal mode.

edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2,edx-solutions/xblock-drag-and-drop-v2