foxy-items-form: Add features to allow this to be used as the cart

[brettflorio]

PLACEHOLDER TICKET. More work on the ticket is needed before work can begin.

Allow foxy-items-form to reflect current cart contents

If FC.json.items is populated, and perhaps if foxy-items-form has a reflect-cart attribute set (language tbd), it should populate with the current cart. That'd allow this component to be used in a huge number of additional circumstances.

Additionally, quantity changes at that point should actually update the cart.

This may also require optionally hide/display the frequency modification and add-to-cart button, as it might not make sense in that context.