User Story 43, Admin enables a merchant account

[iandouglas]

As an admin merchant
When I visit the merchant index page
And I click on a "enable" button for a disabled merchant
I am returned to the admin's merchant index page
And I see a flash message that the merchant's account is now enabled
And I see that the merchant's account is now enabled
This merchant can now log in

DO THESE STORIES FIRST:

• User Story 41, Admin visits 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 #78

child of #125

depends on #75