Better balance between different users for song playtime

[Evanaellio]

If a user has longer songs and another one shorter ones, in the long run the listen time will be unbalanced. In order to avoid this, it would be interesting to track time played for each user and occasionally skip their turn if they have used more time.

We could also initialize subsequent playlists by analyzing the state where the playlist stopped (which users were active and not yet played in rotation) in order to balance rotations across different playlists.

/!\ If someone uses very short videos, they shouldn't be able to play many of them without anyone else being able to play music (maybe count with a minimum time ?).