Allow Replace Player to work with Player 3 and Player 4

[ghost]

In Free-For-Alls and Multi-Battles, the "replace player" feature doesn't work if you're Player 3 or Player 4. This is because replace player was only coded for P1 and P2. So, I added P3 and P4 to make the feature work for Players 3 and 4.

Proof-of-concept replay: https://sheeplol.neocities.org/ffareplaceplayer/

I tested this in my own client and local server. One Free-For-All battle and one Singles Random Battle. From my testing, this is the correct implementation and doesn't break anything.

There could be a way to write this in a better way, but this seems to be sufficient at least.

[pyuk-bot]

This is still a triple nested ternary. None of those parentheses are doing anything to the order of operations; the line would work just the same without any of them.
Anyway,

Suggested change

	var side = (room.battle.mySide.id === room.battle.p1.id) ? 'p1' : (room.battle.mySide.id === room.battle.p2.id) ? 'p2' : (room.battle.mySide.id === room.battle.p3.id) ? 'p3' : 'p4';
	var side = room.battle.mySide.sideid;

Although, since this value is only even used in one place, you can just remove this assignment entirely and just reference room.battle.mySide.sideid directly there instead.