[prediction-markets] Allow metadata changes in pending advised markets

[maltekliemann]

The purpose of this is to allow users to make changes according to the Advisory Committee's comments.

[maltekliemann]

Just a couple of notes:

• To prevent people from "front"-running approve calls with metadata edits, markets should be non-editable by default. When an advisor requests a change, the market id is pushed onto a set which contains all markets which may be edited. After one edit, the market is removed from that set.
• Maybe the advisor allowing the edit should be able to specify a reason for the edit request.
• Advisors themselves should be allowed to edit markets, but the advisory bond should be unreserved when this happens (in order to prevent any animosities if the market is edited in a way that the user doesn't like)
• Question: How does editing markets (particularly having advisors edit user markets) interact with the creator_fee, which allows the market creator to profit from redeemed tokens (which makes being a market creator quite profitable)?

[maltekliemann]

Some more points:

• We also need to be able to change assets (for example, someone might forget to provide a catch-all asset). The best way to approach this is probably to define a function edit(...) which takes virtually the same parameters as create_market to allow changing every aspect of the market by overwriting all attributes. The frontend team considers that to probably be the best solution for their side, as well.
• This PR should also increase the slash amount of reject_market to a fairly high value.