Adding the feature for credit card reuse when an admin adds an order for a customer.
1) Reusing the store-side code for rendering the cards. JS copied from the store side version.
2) Changed admin's payments controller such that the user's cards are available to the view,
and when one such card is chosen, the correct params are passed from view.
Moving code that's common to both store and admin side into one file.
Re-making the commit 42c388b3fb08dda36ffde0828f05ecf9fb0472f1 for spree_reuse_credit_card.
(filter out expired and duplicate credit cards for reuse)
Making minor ellie-related changes to _existing_cards.html.erb.
Minor rearranging.
Moving .css to .scss. Also included some styles for showing new credit card form on the admin site.
Minor styling improvement.
fixed unique check for reusable credit cards so it actually takes last four digits, exp month, and exp year into account