Improve notebook drag and drop experience

[roblourens]

• Dropping between cells should always insert between those cells. That is not how it works now due to how the cell padding is distributed.

• Tweak the insertion indicator position

• Don't show the full markdown preview while dragging

• Fix drag to scroll over cross-domain iframe webview widgets

• Can't easily grab around markdown cell fold chevron

• Dragging markdown cell in edit mode doesn't work

• Dragging over output doesn't show drop indicator - move dragover handler to list container

• Persist markdown edit state across dragging

• Select cell when drag starts?

• Drag to duplicate cell