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.
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