User Story 13, User can log out

[memcmahon]

As a registered user, merchant, or admin
When I visit the logout path
I am redirected to the welcome / home page of the site
And I see a flash message that indicates I am logged out
Any items I had in my shopping cart are deleted

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 #262

child of turingschool-projects#274

[bplantico]

Learned about sessions and passing info around with those. Thanks James!