User Story 68, EXTENSION: Admin can fulfill order items on behalf of a merchant

[BrianZanti]

As a merchant
When I visit an order show page from my dashboard
For each item of mine in the order
If the user's desired quantity is greater than my current inventory quantity for that item
Then I do not see a "fulfill" button or link
Instead I see a big red notice next to the item indicating I cannot fulfill this item

DO THESE STORIES FIRST:

• User Story 53, Merchant fulfills part of an order
• User Story 54, Merchant cannot fulfill an order due to lack of inventory
• User Story 31, All Merchants fulfill items on an order

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 #210

depends on #209

depends on #236