Create layout for checkout pages and abstract layouts into base and sub templates

All layouts now inherit from layouts/base.html.erb, which defines yield blocks for different areas of the page. 

`layouts/admin` and `layouts/application` have been refactored to use the content_for blocks instead of duplicating code. 

Added a new `layouts/checkout` for the checkout pages, which only includes the necessary partials, and updated checkout_confirmation to do the same.