Chart: Improve render performance

[jsnmoon]

This PR refactors the Chart component to be a PureComponent. This prevents unnecessary re-renders when mousing over the chart bars (and triggering a re-render of each <Bar /> in addition to the new <Tooltip />).

Testing instructions

1. Spin up this branch locally.
2. Navigate to /stats/day and select your site of choice.
3. Open your React Devtools and enable Highlight Updates in its configuration.
4. Mouse over the Chart bars and ensure that only the chart container and the tooltip elements are updated.
5. Repeat steps 2~4 in production; you'll notice that the chart bars are updated in addition to the chart container and the tooltip elements.

[matticbot]

Test live: https://calypso.live/?branch=improve/chart-component-render-performance

[ghost]

That's a great PR description, thank you so much for your effort!

Generated by 🚫 dangerJS

[blowery]

LGTM. No more light show.