DO NOT MERGE: Update ProductPage to update quantity sha on quantity change

This is a rough draft to update the quantity sha on change of that field to prevent failed cart validation. The issue is the ajax request won't get back soon enough if a user immediately clicks the Add To Cart button. I think disabling the add to cart button on keyUp or keyDown of that field, then re-enabling it on success of the sha update would be a possible solution to the delay.

Thoughts?