User Story 67, EXTENSION: Admin can manage items on behalf of a merchant

[memcmahon]

As an admin user
When I visit a merchant's profile page
I can click on the merchant's items link
And have access to all functionality the merchant does, including
- adding new items
- editing existing items
- enabling/disabling/deleting items

All content rules still apply (eg, item name cannot be blank, etc)

DO THESE STORIES FIRST:

• User Story 44, Merchant Items Index Page
• User Story 48, Merchant adds an item
• User Story 50, Merchant edits an item

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 #201

child of #274

depends on #220

depends on #216

depends on #214