Avoid creating a container element for each tooltip

[nawatts]

Currently, the TooltipAnchor component attaches a container element to the body and render its tooltip into that container.

gnomad-browser-toolkit/packages/ui/src/tooltip/TooltipAnchor.js

Lines 23 to 34 in 6ed158a

	constructor(props) {
	super(props)
	this.containerElement = document.createElement('div')
	}
	componentDidMount() {
	document.body.appendChild(this.containerElement)
	}
	componentWillUnmount() {
	document.body.removeChild(this.containerElement)
	}

This affects performance when rendering many elements with tooltips, such as some plots. Preferably, all tooltip anchors could share one container.