User Story 12, Link to Checkout

[memcmahon]

As a visitor
When I have items in my cart
And I visit my cart
I see a button or link to Checkout
When I click that button, I am taken to the new order page

DO THESE STORIES FIRST:

• User Story 5, Cart Creation

Checklist:

• base tests are written
• base code is written to pass tests
• edge case ("sad path") tests written (if required)
• edge case code written (if required)
• code is tested in development
• code is reviewed
• branch is merged
• all tests still pass

child of #32

child of #37

depends on #30