Enable negative margin when creating an offfer

[nghiacc]

Some promotional offers may offer negative margin. This needs to be enabled for both Buy and Sell offers.

The copies for margin also need to be simplified as follows (replace the existing copies):

Buy offers:
You buy 5% above market price (or 5% loss).
You buy 5% below market price.
You buy at market price.

Sell offers:
You sell 5% above market price.
You sell 5% below market price (or 5% loss).
You sell at market price.