User Story 20, User adds an item to the cart

[memcmahon]

As a visitor or registered user
When I visit an item's show page from the items catalog
I see a link or button to add this item to my cart
And I click its link or button
I am returned to the item index page
I see a flash message indicating the item has been added to my cart
The navigation bar increments my cart counter

DO THESE STORIES FIRST:

• User Story 16, Item Show Page

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#254