Add field for Coupon Codes to billing info!

[cheyenneweaver]

• add field to db at billing info
• show this in email
• AND on admin dash
• AND we should probably add it to the confirmation page also --> "We received your offer code"

[cheyenneweaver]

@ddgriffin - I added the front-end into the _credit_card_form.html.haml It needs to be hooked to the database

[cheyenneweaver]

@ddgriffin - Scott helped me add the class :coupon to the event model and to the database. It's showing up on the Admin dashboard, confirmation page and I wrote a very vague reference to it in the email. We should be good!