User Story 48, Merchant adds an item

[memcmahon]

As a merchant
When I visit my items page
And I click on the link to add a new item
My URI route should be "/dashboard/items/new"
I see a form where I can add new information about an item, including:
- the name of the item, which cannot be blank
- a description for the item, which cannot be blank
- a thumbnail image URL string, which CAN be left blank
- a price which must be greater than $0.00
- my current inventory count of this item which is 0 or greater

When I submit valid information and save the form
I am taken back to my items page
I see a flash message indicating my new item is saved
I see the new item on the page, and it is enabled and available for sale
If I left the image field blank, I see a placeholder image for the thumbnail

DO THESE STORIES FIRST:

• User Story 44, Merchant Items Index 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 turingschool-projects#221

child of turingschool-projects#274

depends on turingschool-projects#220