User Story 66, EXTENSION: Admin enables a user account

[memcmahon]

As an admin user
When I visit the user index page
And I click on a "enable" button for a disabled user
I am returned to the admin's user index page
And I see a flash message that the user's account is now enabled
And I see that the user's account is now enabled
This user can now log in
This user's city/state and orders should be included in all statistics.

DO THESE STORIES FIRST:

• User Story 55, Admin User 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-projects#201

child of turingschool-projects#274

depends on turingschool-projects#207