Show profitableness of arbitration in the UI

[edmundedgar]

Arbitration is only profitable if the arbitration fee is lower than the amount you expect to gain from the question reward and other people's bonds. We currently display the arbitration fee with the "request arbitration" link, so it's possible to manually calculate whether it will be profitable. However, we should probably add a warning or some other UI display if you try to request arbitration at a rate that will be unprofitable.