User Story 59, Admin can downgrade a merchant to regular user

[iandouglas]

As an admin user
When I visit a merchant's dashboard ("/admin/merchants/6")
I see a link to "downgrade" the merchant's account to become a regular user
The merchant themselves do NOT see this link
When I click on that link
I am redirected to ("/admin/users/6") because the merchant is now a regular user
And I see a flash message indicating the user has been downgraded
The next time this user logs in they are no longer a merchant
All items for sale by this user are disabled
Only admins can see the "downgrade" button
Only admins can reach any route necessary to downgrade the merchant to user status

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 #59

child of #125