User Story 55, Admin User Index Page

[BrianZanti]

As an admin user
When I click a new "Users" link in the nav (only visible to admins)
Then my current URI route is "/admin/users"
Only admin users can reach this path.
I see all users in the system who are not merchants nor admins.
Each user's name is a link to a show page for that user ("/admin/users/5")
Next to each user's name is the date they registered
Next to each user's name is a button that says 'Upgrade to Merchant'

DO THESE STORIES FIRST:

• User Story 5, Admin Navigation

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 turingschool-projects#274

depends on turingschool-projects#268