Rework course overview drag and drop.

Failed drags bring the element back to where it started; elements are
locked to the y-axis, states are represented as CSS classes for
styling, elements can be dragged across section/subsection borders,
and dragging to the top/bottom of a section Just Works™.

Needs some styling love to give visual representation of where the
dragged element will drop, though.

TODO: It'd be good to have auto expand/collapse for subsections.
Add auto-expand to overview lists.
Make all of section/subsection/unit draggable.