Convert checkout page to HAML

[#40820419]
Add checkout page background and fix table style

Also remove white rounded border halfway through the page

[#40820419]
Remove +/- tabs from checkout page

[#40820419]
Complete layout & styling for checkout page

- Convert to HAML
- Swap payment information and shipping address, so sales tax adjustment will be shown sooner
- Change button color on hover
- Change button wording from 'Confirm' to 'Complete Purchase'
- Add prompts to state and expiration month and year

[#40820419]
Darken text in form fields

[#40820419]
Ensure background image hugs bottom of page

[#40820419]
Move payment form errors to correct location

[#40820419]
Remove PayPal dependencies

- Comment out paypal javascript
- Remove Paypal validation requirements

[#40820419]
Better empty cart error message and display

[#40820419]
Show submit button as disabled while purchasing

[Finishes #40820419]