Restore an item's body to an earlier version

Restore one item's or expert's body to an earlier version from list_versions (its cells too, where the snapshot kept them). The current body is snapshotted first, so a restore is itself undoable. When the person, or a collaborator, edited the body in Roma after that version, the call answers needsConfirmation and names those later edits instead of restoring; the same call with acceptLaterEdits: true, after the person agreed to lose them, restores. Writes made over this connection never ask: undoing them is what the tool is for.