ZIP-0 Only allow one dispute in prediction-markets

[Chralt98]

The point of this section of ZIP-0 was that the dispute extrinsic is used to pass control to the MDM. From there on, all further interaction should go via the MDM. So the whole disputes parameter here should go.

There should only be one dispute regardless of what the MDM is, so the check shouldn't go into the MDM, but into prediction-markets (or rather, the whole Disputes object should not map to a vector of disputes).

simple-disputes does not work with the current implementation, when we only allow one dispute. So there needs to be a possibility to dispute inside the simple-disputes MDM (rather than in prediction-markets).

Many things like Disputes and the dispute bond managing should go into simple-disputes and removed from prediction-markets.

After the first dispute is called, everything should happen inside the MDMs.

[Chralt98]

Strongly linked to #916

[Chralt98]

Fixed by #938