getAvailableItems() should use a notifier

[michaelfig]

In the card store UI (web inspector console), I'm seeing a lot of (once per second or so) calls to getAvailableItems. It's generating quite a bit of traffic to the chain. The dapp does eventually create an offer, though it's 0.00001 RUN for 1 card.

@katelynsills says:

It needs a way to know what the current items are, so maybe the contract needs to provide a notifier