Add product foreign keys to salestrans and quoteline

Some of this is harder than it seems because of old description-only lines.

But, without fixing this the current "fake" foreign key is useless anyway. In fact, what we have done is worse than nothing because it tempts you to expand from quote line to proditem and then the whole thing crashes. So, lets fix this properly.

Also, you cannot expand from salestrans to proditem at the moment - something that you would almost certainly want to do (probably not as urgent as quoteline but same problem so solve together if we can.)