Allow re-ordering categories

[DanielOaks]

When we import the sample categories Poses, Faces, Animals, and Hands & Feet, they end up slightly mis-ordered:

This brings up a good point – we should be able to drag categories on the Admin page to re-order them on the index:

[DanielOaks]

The API call was added in drawref/drawref-backend-js@50303a1

[DanielOaks]

We can move the category list into its own component, and have it output events for dragging the categories into a new order. But for now, we've just added a button to make the API call with a pre-determined order: 7fbee21