Add some more validation to Trade form
Add some more validation to Trade form

I didn't test it, and it was done rather hastily, but I think it might
work. It makes sure that none of the stocks are already involved in a
trade. Also, the formatting could be better, and it might be slow. We'll
see.

ddsnowboard/FantasyStocks,ddsnowboard/FantasyStocks,ddsnowboard/FantasyStocks