User Story 31, All Merchants fulfill items on an order

[iandouglas]

When all items in an order have been "fulfilled" by their merchants
The order status changes from "pending" to "packaged"

DO THESE STORIES FIRST:

• User Story 53, Merchant fulfills part of 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 #125

depends on #61