CSFloat: Improve 'cart' experience

[GODrums]

The cart on CSFloat is kinda useless for most situations. But since the rate limits by CSFloat got harsher and you cannot buy more than 8 items successively, the cart has to be used when buying larger quantities of items such as capsules or cases.

Useful features:

• total Buff price/price difference
• list amount of items in cart

Handle errors:

• "Item {item_id} has been purchased already" -> fix by either marking the sold item or automatically removing it from the cart and still buying every other item that's in the cart.