Rename apply_gift_code to apply_gift_codes
Modify checkout controller to utilize the accepts multiple gift codes

-Modify overrides to render an equvilant partial.
-Change params from order[gift_code] to gift_code
Refactor apply_gift_codes to not use @order.gift_code

-Remove attr_accessor on order
Add more exhaustive specs for multiple gcs
Add index to code on spree gift cards.

We regularly use Spree::GiftCard.find_by(code: code) in our code, this
will help optimize this.
Minor spec tweaks.
Change the way errors are displayed

Apply gc errors to the order. Do not show success flashes.
Extract logic into a module
Refactor apply_gift_codes
Merge pull request #42 from Senjai/2-1-multiplgcs

Expand gift code application logic for multiple gift cards.