[PM] Add the ability to flag invalid markets

[lsaether]

Users should be able to flag markets that were created permissionlessly. Flagging requires a small ZTG bond, and puts the market into a queue. The markets with the most flags move to the top of the queue, and their validity can be decided on by the Advisory Committee. If the market is decided to be invalid, then all flagging users get their deposits back and the original market creator forfeits their ValidityBond to the treasury.

[sea212]

I think it would be nice to reward those who flag markets correctly and risk reducing their ZTG bond. Maybe we can use part of the ValidityBond to reward those who flagged it and the other part goes into the treasury.

[sea212]

We also have to define precisely and transparently and communicate which properties render a market as "invalid"