feat(demo): visx rebrand, move to github pages

[hshoff]

This was a part of the v1 release. Been delayed to get the PR out.

🐛 Bug Fix

• fix(demo/tooltip): fix z-index for tooltip in portal demo. fixes [demo] all demos that use TooltipWithPortal have broken z-index #850
• fix(demo/tooltip): use pointerMove instead both mouseMove & touchMove.fixes [tooltip] positioning breaks on swiping on a mobile device / tab #861
• fix(demo): use named default components for next.js fast refresh

🏠 Internal

• feat(demo): move to github pages
• fix(demo): add html lang attr for a11y
• fix(demo): add aria-label to footer logos for a11y
• deps(demo): bump next.js 9.4 => 9.5
• deps(demo): remove react-ga
• deps(demo): bump yarn.lock

[coveralls]

Pull Request Test Coverage Report for Build 344688573

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 11 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.5%) to 60.095%

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Files with Coverage Reduction	New Missed Lines	%
packages/visx-xychart/src/utils/getScaledValueFactory.ts	1	69.23%
packages/visx-xychart/src/utils/getBarStackRegistryData.ts	2	82.61%
packages/visx-xychart/src/components/series/private/BaseBarGroup.tsx	3	75.83%
packages/visx-xychart/src/components/series/private/BaseBarStack.tsx	5	77.42%

Totals
Change from base Build 263:	-0.5%
Covered Lines:	1549
Relevant Lines:	2445

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💛 - Coveralls

[hshoff]

Lighthouse audit (desktop) for homepage:

[williaster]

Thanks for landing this! 🚀