Add puzzle rating on/off toggle

[NewSupKaizo55]

There's a useful toggle on the Lichess website that lets the user enable or disable the puzzle rating.

Sometimes I like to solve puzzles, even when I'm aware that I can't concentrate enough. This toggle lets me do puzzles without the risk of losing too much rating when it's very likely to perform poorly - without the need of logging out.

In my opinion this would be a neat addition to the new app.

[veloce]

This is quite a low priority on the app, because it would add a lot of complexity, as puzzles are downloaded by batches and work offline.