[simple_disputes] Disputes resolve to last dispute without global dispute

[maltekliemann]

On a market with simple_disputes as DRM, I can ping-pong disputes back and forth until MaxDisputes is reached. But then, instead of creating a global dispute, the market just resolves to the last reported outcome after DisputePeriod. It doesn't seem like global disputes are implemented, right now (or am I missing something?): https://github.com/zeitgeistpm/zeitgeist/blob/9ca3d7e71c42fc1904d7ecfc4aa88003e19a8a5c/zrml/prediction-markets/src/lib.rs#L818-L828

If there are three or more outcomes, this allows someone using two accounts to "game" the outcome of the market to an outcome of their choice.

There are two possible solutions:

• Implement token holder vote when MaxDisputes is reached
• Disable simple_disputes (not pretty, since then we've only got the authorized DRM, which requires a great deal of trust in the oracle and authorized arbiter)

[Chralt98]

Don't forget the extrinsic description in the PR.