Qty selector

Update to the qty selector for added keyboard accessibility and clarity.

- Remove "x" from quantity selector in the cart (and ajax cart)
- Quantity add/remove buttons are now `button` elements, rather than spans so they are added to the tab index
- Removed unneeded inline svg backgrounds for –/+ icons in favour of unicode text
- Centered quantity selector in the cart layout

[Demo here](https://carson-dev-shop.myshopify.com/products/canon-eos-rebel-t3-12-2mp-dslr-camera-with-18-55mm-lens-camera-case-memory-card?variant=687054091)

cc/ @stevebosworth @shopify/fed