Better and more consistent keyboard tabbing/focus behavior across browsers

[edelstone]

Keyboard tabbing through the site, and particularly the color table after copying a hex color, is not standard across browsers.

The way Chrome treats tabbing after a copy seems to be the most ideal way to me: After copying, the focus appears to stay on the copied cell even though the copy/checkmark button disappears. If you tab again from this position, the next table cell is focused. This seems logical to me, but it's not the current behavior in FF or Safari on the Mac, and perhaps other browsers too.

Would be cool if the keyboard tabbing was standardized around this. We had a PR that initially played with this a bit (#23) but it was abandoned. There might be some useful comments/code there.