User Story 24, User can manipulate quantities in their cart

[iandouglas]

As a visitor or registered user
When I have items in my cart
And I visit my cart
Next to each item in my cart
I see a button or link to remove that item from my cart
- clicking this button will remove the item but not other items

I see a button or link to increment the count of items I want to purchase
- I cannot increment the count beyond the merchant's inventory size

I see a button or link to decrement the count of items I want to purchase
- If I decrement the count to 0 the item is immediately removed from my cart

DO THESE STORIES FIRST:

• User Story 21, User views their cart show page with items in the cart

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 #100

child of #125

depends on #98