Improve Add to Cart button for better user feedback and in-situ quantity editing

[benmarcotte]

This is the master ticket for implementing optional, online grocery store style Add to Cart buttons.

The objective is to have the Add to Cart buttons show how many of each particular item you have already in your cart, and an optional feature to let you edit the quantity in your cart directly from the Add to Cart button, instead of needing to go into each individual product page. This feature can be found on many online grocery websites already.

Wiki page for this feature

Tasks to complete project:

• [ ] Implement feature that has the button show how many of a given product is in the cart
• Needs to work with items sold by the unit as well as items sold by a specific unit of weight/quantity
• Should this be optional? Seems like an unarguable, universal improvement
• [ ] Implement optional direct-on-button amount modification
• Should this be configurable store-wide or by each product (or both)?
• This would need to disable the pop-up that appears when Add to cart is pressed

[bgambocjaviniar]

moving to do because owner for the PR finished their internship, therefore, anyone can pick up or create a new PR.