Render all adjustments on the cart page
[frontend] Move coupon code promotion feature to 'coupon_code_spec'

Having a 500000000000 line file to test all the promotions would be no fun. Let's separate it out sensibly.
Make automatic ItemTotal rule promotions tick over correctly
Correct stubs in item_total_spec
Define PromotionRule#applicable? to determine if a rule is applicable to an item that has a promotion adjustment already applied

This stops things like ItemTotal rules from being checked against line items and shipments
Define applicable? in PromotionRule stubs for promotion_spec
Do not count an adjustments already applied for an order in the promotion's usage limit check

AKA: Ohhhhh, so *that's* what that did.