improve broadcast leaderboard functionality

[Arihol]

Broadcasts have an automatic leaderboard function, which generates standings based on game results across the broadcast.

It is designed to work in Round-Robin formats, where all the participants are present in all rounds.

However, in some tournaments, some rounds are “worth” more than others. For example, in the following broadcast

https://lichess.org/broadcast/2022-vugar-gashimov-memorial/q2H7Oi6j

the rapid portion is worth twice as many points as the blitz section. This means we had to generate a manual markdown table instead of using the native leaderboard, even though the format (SRR&DRR) is ideal for it.

It would be nice if there was a factor in the round settings you could set manually. For example, if you set the factor as 2, a win is worth 2 points, a draw 1 point and a loss 0 points. Similarly, a factor of 0 could be used for tiebreaks, with the results indicated inside brackets: 12 (3), 12 (0) etc.