[simple-disputes] Implement global disputes

[sea212]

Currently simple disputes accepts N disputes from anybody who is willing to place a stake behind the dispute that grows after each dispute. The Nth dispute will currently be taken as the correct answer.

The dispute mechanism should be extended by another final dispute step: The global dispute. During the global dispute anybody can cast a vote that is weighted by the tokens staked with the vote.

Some open questions:

• Should the last dispute automatically trigger a global dispute or should this be an additional step?
  • In case of an additional step: What should happen with the stake of the person who triggered the global dispute?
• What happens with the stake from the losing positions? Suggestions: Distribute to correct voters, burn, treasury (can be a combination)