Implement Drag and Drop re-ordering

[damonhook]

Currently you can only re-order elements using an up/down button. We should instead use a drag-and-drop interface using this library: https://github.com/atlassian/react-beautiful-dnd

[damonhook]

I got it mostly working, however, it conflicts with the 'react-swipeable-views' library used to enable touch swiping for tabs.

This is because it creates a nested scroll container which are not currently supported by react-beautiful-dnd. I am going to put this back onto the back-burner for now, and get back to some more important issues