Allow add to cart to bump up qty

Summary: Refs T1507

Test Plan:
* you can't use your script for this
* git pull blendtec's origin develop
* cd Plugin/Cart
* git checkout feature/orders
* arc patch D### --nobranch
* go to a product page, add  to cart, go back to product page add to cart again, should be 2 of them now
* if it all works accept said diff
* then cd ../..
* git submodule update
* git status you should be back to normal, maybe

Reviewers: #everyone, jslemboski

Reviewed By: #everyone, jslemboski

Subscribers: jslemboski, lwhite

Maniphest Tasks: T1507

Differential Revision: https://dev.blendtec.com/D967