"Up" arrow to edit last added done

[aduth]

Slack has a nice behavior where adding a message, then proceeding to press the "Up" arrow, gives you the opportunity to edit this message. This is useful in situations where one wants to make revisions after having already submitted the message, without needing to leave the keyboard flow.

Dones should support this behavior: Pressing the "Up" arrow from the dones input should start editing the most recently added done.

Optionally, pressing Up from the beginning of the input of a done being edited should transition editing to the prior done.