Refactor scoreboard

[Bamcane]

See also:
teeworlds/teeworlds#354
teeworlds/teeworlds#1828
teeworlds/teeworlds#3300

[Bamcane]

Now, we have done half of this issue in 48d1901

[Bamcane]

I think we could merge the statsboard and the scoreboard. The new name could be playerboard.