[PM] add collaborative liquidity provision

[sea212]

With https://github.com/zeitgeistpm/zeitgeist/pull/711 pools are now closed until the market starts. A nice improvement would be to allow to collaboratively add liquidity without a lower bound to that pool until the market opens. We should decide if there should be a lower bound (maybe configurable per market), that when not surpassed will result in the deletion of the market and the pool (interest too low). This is the case with Rikiddo currently, any market that does not have sufficient liquidity to start with is discarded, because the providers either are not interested in the outcome or think that the outcome is already certain.

[maltekliemann]

Yeah, sounds good. Do you see any problem with letting the crowdfunding run past market open? My thinking is: The market creator decides to crowdfund the pool. There's no harm done if that takes longer.