Lazy-load charts on scroll-into-view

[kenkunz]

TBD whether this is needed.

We currently load ChartIQ-based charts client-side on the pair/index page. This results in multiple requests to /candles and /xyliquidity APIs. We could reduce server load by waiting to load the chart data until the charts are scrolled into view.

This would have a (relatively small) negative impact on the user experience – users who are trying to view the charts will experience more latency until the charts are loaded.