Improve readability in FlexiRate calculator labels

Based on discussion in https://github.com/bonobos/spree/pull/49 regarding #5007. Changes the FlexiRate promotion labels to read "First Item Discount" and "First Adjustment Discount". Because the labels are also used in the shipping FlexiRate calculator, this change was a bit more complicated than just a simple change to en.yml. But, I think it improves readability and intent in the UI as well as the code.