`comments-time-machine-links` should only appear on hover #2131

Follows https://github.com/sindresorhus/refined-github/pull/1863

I find the icon added next to each link ugly, especially like https://github.com/sindresorhus/refined-github/blob/master/readme.md in blocks of text where it's rather distracting.

Should we show this only when hovering the link? With position: absolute so it doesn't change the layout. Better yet, perhaps we should just show a tooltip that invites the user to alt-click the link.