Zerosum DKP was requested via discord.

We have some settings for it, but no implementing code yet.

Additional context
Requested by Rashaka from [Critical Mass] on [Atiesh]

@lantisnt my idea for implementing this:

• Use different kind of award entry (since we need to have all players that are credited in there)
• Add new mutator that decreases winner's DKP and awards that DKP to the rest
• Figure out rounding, how much DKP disappearing is acceptable?
• What is the inflation setting?

Assumptions:

• The raid itself is not in the ledger, therefore we must store all players with the event.
• Enabling / disabling zero sum (even in the past) does not affect existing rewards.

Possible improvements:

• If we store the raid in the ledger we can track who's actually there (attendance records)
• Storing the raid once is cheaper than doing it for every item that is awarded.

Postpone until #43 is done and use the fact of tracking players

#43 Finished.
Resolves #19

Blocked by #59

/related #19