Improve readability in FlexiRate calculator labels

Based on discussion in #5659. Makes the following label changes to the FlexiRate calculator:

* "First Item Discount" --> "First Item"
* "Additional Item Discount" --> "Additional Item" 

Using "Discount" made sense in the context of the FlexiRate shipping calculator, but it did not make sense in the FlexiRate promotional calculator. These labels are more flexible and leave it up to the user to infer the context.