Drag and drop support on the course outline page.
Studio: re-implemeting visual styling for drag and drop:

* adding back in drag and drop indicators to outline DOM
* correcting visual display of drag action in outline UI
* adjusting styling of outline drag and drop indicators
* adjusting styling of drag and drop was-dropped state
resolving mid-drop unit marign issue in Firefox
Increase "fudge" factor for dragging to top of list with only 1 element.
Merge pull request #4605 from edx/christina/drag-drop-outline-3

Christina/drag drop outline 3