User Story 46, Merchant enables an item

[BrianZanti]

As a merchant
When I visit my items page
And I click on an "enable" button or link for an item
I am returned to my items page
I see a flash message indicating this item is now available for sale
I see the item is now enabled

DO THESE STORIES FIRST:

• User Story 44, Merchant Items Index 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 #221

child of #274

depends on #220