Enigma - Drag and drop question rearrangement

[FrenjaminBanklin]

As of the fixes contained in #546, Enigma allows questions to be moved around within their categories via some buttons in the question editor. There is a desire to supplement this with the ability to simply drag and drop question blocks with and between categories.

This will almost certainly require some Angular module(s), such as https://github.com/luckylooke/dragular.