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.

package.json

@@ -144,6 +144,8 @@
     "react": "^15.5.0",
     "react-autosuggest": "^7.0.1",
     "react-datetime": "^2.6.0",
+    "react-dnd": "^2.5.4",
+    "react-dnd-html5-backend": "^2.5.4",
     "react-dom": "^15.5.0",
     "react-frame-component": "^1.0.3",
     "react-hot-loader": "^3.0.0-beta.7",
@@ -152,7 +154,6 @@
     "react-router-dom": "^4.2.2",
     "react-router-redux": "^5.0.0-alpha.6",
     "react-sidebar": "^2.2.1",
-    "react-simple-dnd": "^0.1.2",
     "react-sortable": "^1.2.0",
     "react-split-pane": "^0.1.66",
     "react-toolbox": "^2.0.0-beta.12",