Add decoding=async to images as appropriate in Shop

[chrishtr]

There is a new attribute available on tags called "decoding", which specifies a developers
preference for synchronous or asynchronous decoding. the default is "auto", which means the user
agent may choose its own preference. For good performance it should probably be recommended to
use decoding=async on images that are content (images of clothing) as opposed to expected to be
strongly connected to the brand of the site or its navigation (logo, UI icons).

Spec:

https://html.spec.whatwg.org/#decoding-images

Safari will support this feature in the next release. Chrome is likely going to support it in version 65.

[keanulee]

Made a branch for this at https://github.com/Polymer/shop/tree/async-img and deployed a test version to https://async-img-dot-polymer-shop.appspot.com/