Refactor leaderboards

[tmalahie]

Many pages in the sites show the same kind of table for leaderboards:

• https://mkpc.malahieude.net/bestscores.php
• https://mkpc.malahieude.net/challengeRanking.php
• https://mkpc.malahieude.net/classement.global.php
• https://mkpc.malahieude.net/ranking-forum.php
• https://mkpc.malahieude.net/ranking-advent.php
• https://mkpc.malahieude.net/localscores.php

Problem: for now all those pages have lots of copy & pastes, I never took the time to refactor it properly.
Since it all shares a lot of logic, it could be cool to refactor all that is common somewhere. The file utils-leaderboard.php could be a good place to put all the refactor code.