Remove react-dnd wrapper lib.

`react-simple-dnd` is just a wrapper for `react-dnd`, and it doesn't
support React 16. This PR changes to using the underlying library
directly.

src/components/UI/dndHelpers.js

@@ -0,0 +1,51 @@
+import ReactDNDHTML5Backend from 'react-dnd-html5-backend';
+import { DragDropContext as ReactDNDDragDropContext, DragSource as ReactDNDDragSource, DropTarget as ReactDNDDropTarget } from 'react-dnd';
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export const DragSource = ({ namespace, ...props }) => {
+  const DragComponent = ReactDNDDragSource(
+    namespace,
+    {
+      beginDrag({ children, isDragging, connectDragComponent, ...ownProps }) {
+        // We return the rest of the props as the ID of the element being dragged.
+        return ownProps;
+      },
+    },
+    (connect, monitor) => ({
+      connectDragComponent: connect.dragSource(),
+    }),
+  )(
+    ({ children, connectDragComponent }) => children(connectDragComponent)
+  );
+
+  return React.createElement(DragComponent, props, props.children);
+};
+DragSource.propTypes = {
+  children: PropTypes.func.isRequired,
+};
+
+export const DropTarget = ({ onDrop, namespace, ...props }) => {
+  const DropComponent = ReactDNDDropTarget(
+    namespace,
+    {
+      drop(ownProps, monitor) {
+        onDrop(monitor.getItem());
+      },
+    },
+    (connect, monitor) => ({
+      connectDropTarget: connect.dropTarget(),
+      isHovered: monitor.isOver(),
+    }),
+  )(
+    ({ children, connectDropTarget, isHovered }) => children(connectDropTarget, { isHovered })
+  );
+
+  return React.createElement(DropComponent, props, props.children);
+};
+DropTarget.propTypes = {
+  onDrop: PropTypes.func.isRequired,
+  children: PropTypes.func.isRequired,
+};
+
+export const HTML5DragDrop = component => ReactDNDDragDropContext(ReactDNDHTML5Backend)(component);