Lights on by default

Changed

• lights now defaults to on: a request without the parameter returns the image with the headlights and tail lights on, wherever the image has a lit variant. Until now it defaulted to off.
• lights=off returns the unlit image, exactly as before. Image links you already hold keep returning the image they were signed for.
• An image without a lit variant comes without lights and says nothing about it; only an explicit lights=on still adds errornote L26. A key whose lights option is switched off keeps getting the unlit image, without an error.