Lichess Puzzle algorithm

@Toadofsky does it matter how many puzzles you have to have done for them to change? I've done over 15,000 so its all the same to me. And also why did the feature where you could select the difficulty of puzzles get removed?

@Doofenshmirtz, I start to get the easy ones wrong because I get bored of them and tend to lose focus

@V2chess The difficulty selector was removed because as the site upgraded (not just V2, but in general upgrades are occurring all the time), the puzzle module was too complex and some code had to be sacrificed.