Replace TagPanel react-dnd with react-beautiful-dnd

This new library handles the simple case of an ordered vertical
(or horizontal) list of items that can be reordered.

It provides animations, handles positioning of items mid-drag
and exposes a much simpler API to react-dnd (with a slight loss
of potential function, but we don't need this flexibility here
anyway).

Apart from this, TagOrderStore had to be changed in a highly
coupled way, but arguably for the better. Instead of being
updated incrementally every time an item is dragged over
another and having a separate "commit" action, the
asyncronous action `moveTag` is used to reposition the tag in
the list and both dispatch an optimistic update and carry out
the request as before. (The MatrixActions.accountData is still
used to indicate a successful reordering of tags).

The view is updated instantly, in an animated way, and this
is handled at the layer "above" React by the DND library.

src/actions/actionCreators.js

@@ -22,6 +22,9 @@ limitations under the License.
  *                    suffix determining whether it is pending, successful or
  *                    a failure.
  * @param {function} fn a function that returns a Promise.
+ * @param {function?} pendingFn a function that returns an object to assign
+ *                              to the `request` key of the ${id}.pending
+ *                              payload.
  * @returns {function} an action thunk - a function that uses its single
  *                     argument as a dispatch function to dispatch the
  *                     following actions:
@@ -29,9 +32,13 @@ limitations under the License.
  *                         `${id}.success` or
  *                         `${id}.failure`.
  */
-export function asyncAction(id, fn) {
+export function asyncAction(id, fn, pendingFn) {
     return (dispatch) => {
-        dispatch({action: id + '.pending'});
+        dispatch({
+            action: id + '.pending',
+            request:
+                typeof pendingFn === 'function' ? pendingFn() : undefined,
+        });
         fn().then((result) => {
             dispatch({action: id + '.success', result});
         }).catch((err) => {