User Story 57, Admin can make a User a Merchant

[BrianZanti]

As an admin user
When I visit a user's profile page ("/admin/users/5")
I see a link to "upgrade" the user's account to become a merchant
When I click on that link
I am redirected to ("/admin/merchants/5") because the user is now a merchant
And I see a flash message indicating the user has been upgraded
The next time this user logs in they are now a merchant
Only admins can reach any route necessary to upgrade the user to merchant status

DO THESE STORIES FIRST:

• User Story 55, Admin User Index Page
• User Story 38, Admin can see a merchant's dashboard

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 #208

child of #274

depends on #207

depends on #228