User Story 41, Admin visits Merchant Index Page

[BrianZanti]

As an admin user
When I visit the merchant's index page at "/merchants"
I see all merchants in the system
Next to each merchant's name I see their city and state
The merchant's name is a link to their Merchant Dashboard at routes such as "/admin/merchants/5"
I see a "disable" button next to any merchants who are not yet disabled
I see an "enable" button next to any merchants whose accounts are disabled

DO THESE STORIES FIRST:

• User Story 39, 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 #227

child of #274

depends on #226