#358 Have dropdown menu for concept and mapping node expansion menus. This was really way trickier than it should have been, fussing with svg and jquery disagreements (so use D3 isntead for all SVG stuff), and remembering to use mouseleave rather than mouseout. The methods called when we click dead-end because the callback used has indeed already been fulfilled. Looking at whether this is a design issue of the REST cache...I suspect it is.