Mobile browsers have problems grabbing ASIN

[razorman8669]

the code
https://github.com/Purse/buywithbitcoin/blob/master/src/content/app/ProductApp.jsx#L34

    const asin = document.querySelector('#addToCart #ASIN').value;

fails on mobile browsers since amazon removes this element when rendering on mobile. we need another way to grab the ASIN so that we can add to cart.

This however, isn't really an issue, since the chrome/firefox extensions cannot (currently) be installed on mobile. However, once firefox allows installing any extension on mobile firefox, we should fix this bug.