Don't execute "Customize Slave Trade" from within <<silently>>, because it doesn't work that way anymore

customizeSlaveTrade.tw wants to use a span element it writes, but is being executed in a <<silently>> block during NGP initialization, and <<silently>> no longer writes the span element to the DOM. Replace <<silently>> with a hidden <div> so it executes just like it used to.

Fixes #1229 (closed)