[prediction-markets] Let market creators manually close markets

[maltekliemann]

This is useful if an information shock occurs that invalidates the market. For example, if the question is "Will X happen in 2023?" and then X happens in January, let's not keep the market open until the end of the year. Let's instead close the market manually and proceed to resolve it.

There are some concerns here. Allowing the market creator to do this gives them the power to hard-rug the market (as opposed to just soft-rugging it by withdrawing liquidity). The solution would be to make this feature only available to the Advisory Committee, instead?

The reported outcome may be disputed. An inconclusive dispute may be used to indicate that the market should reopen.

One cool application is that we could even run markets which are open-ended.

[maltekliemann]

Fixed by #1127, closing this early because GitHub won't let me link this issue.