Add "increase quantity" button

Previously, there was no way for a customer to increase the quantity of items in their basket, which meant they had to explicitly add a product to their basket repeatedly to increase its quantity. Added a new button to the basket that allows the user to increase the quantity of an item in the basket.

https://trello.com/c/dwme2BLI

![](http://www.reactiongifs.com/r/jdhst.gif)