User Story 30, User cancels an order

[BrianZanti]

As a registered user
When I visit an order's show page
If the order is still "pending", I see a button or link to cancel the order
When I click the cancel button for an order, the following happens:
- Each row in the "order items" table is given a status of "unfulfilled"
- The order itself is given a status of "cancelled"
- Any item quantities in the order that were previously fulfilled have their quantities returned to their respective merchant's inventory for that item.
- I am returned to my profile page
- I see a flash message telling me the order is now cancelled
- And I see that this order now has an updated status of "cancelled"

DO THESE STORIES FIRST:

• User Story 17, User Profile Show Page
• User Story 29, User views an Order Show 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 #241

child of #274

depends on #252

depends on #238