Finished projects wait on the canvas to be archived

When the last workstream of a project landed, Obeya used to take the whole project off the canvas on its own, and it was only to be found in the archive. Now a finished project stays in its place, marked done, with an Archive button in its head and in its sheet. One click (with Undo) moves it into the archive, and from there it can be brought back onto the canvas. "Archive finished cards" takes finished projects along too.