User Story 48, Admin disables a merchant account

[megstang]

As an admin
When I visit the merchant index page
I see a "disable" button next to any merchants who are not yet disabled
When I click on the "disable" button
I am returned to the admin's merchant index page where I see that the merchant's account is now disabled
And I see a flash message that the merchant's account is now disabled

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 turingschool-examples#194

child of turingschool-examples#242

depends on turingschool-examples#191