User Story 51, Enabled Merchant Item's are active

[BrianZanti]

As an admin
When I visit the merchant index page
And I click on the "enable" button for a disabled merchant
Then all of that merchant's items should be activated

DO THESE STORIES FIRST:

• User Story 44, Admin Merchant 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 #194

child of #242

depends on #191