User Story 23, User can empty a cart that has items

[memcmahon]

As a visitor or registered user
When I have items in my cart
And I visit my cart ("/cart")
And I click the link to empty my cart
Then I am returned to my cart
All items have been completely removed from my cart
The navigation bar shows 0 items in my cart

DO THESE STORIES FIRST:

• User Story 21, User views their cart show page with items in the cart

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 reviewed
• branch is merged
• all tests still pass

child of turingschool-projects#249

child of turingschool-projects#274

depends on turingschool-projects#247