Implement action undo/redo

[djwhitt]

Implementation ideas:

• Store actions in the DataScript DB but not the serialized game state. This means only actions for the current turn will be undoable.
• Implement undo-attack-tx (dual of attack-tx) that, given an action creates, returns a transaction that undoes it.
• Stochastic attacks, if enabled, may require special handling since allowing them to be replayed makes it easy to cheat by cherry picking good attacks.