User Story 50, Merchant edits an item

[iandouglas]

As a merchant
When I visit my items page
And I click the edit button or link next to any item
Then I am taken to a form similar to the 'new item' form
My URI route will be "/dashboard/items/15/edit" (if the item's ID was 15)
The form is re-populated with all of this item's information
I can change any information, but all of the rules for adding a new item still apply:
- name and description cannot be blank
- price cannot be less than $0.00
- inventory must be 0 or greater

When I submit the form
I am taken back to my items page
I see a flash message indicating my item is updated
I see the item's new information on the page, and it maintains its previous enabled/disabled state
If I left the image field blank, I see a placeholder image for the thumbnail

DO THESE STORIES FIRST:

• User Story 48, Merchant adds 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 #72

child of #125

depends on #67