Also includes RequireJS changes.

Fixes bugs:
STUD-755, STUD-152, STUD-700, STUD-699

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.