Change behavior of joining open slots

[SpacialCircumstances]

Right now:

• Concede defeat + open slot opens the slot again, and a player who joins it can gain/lose rank as normal
• Joining a slot that was forcefully opened due to afk means you gain more rank and cannot lose any

Planned:

• Afk behaviour continues as right now
• A slot that was opened by concede defeat will not give extra rank, but will prevent the late joining player from losing rank

This requires tracking "slot was filled later" as an extra flag besides "filledAfkSlot"