fix(gatsby-theme-blog): Trigger link icon beside Heading on hover

[NickyMeuleman]

Description & motivation

Hovering over where the link icon would be now triggers showing it.
This brings the behaviour more in line with gatsby-remark-autolink-headers.

Mouse users often don't hover over the text of a header first to click the link.
I know this is a very minor change, but it relieves a minor inconvenience of the icon not showing up when you expect it to.

If not set pointer-events defaults to auto, which is the same as visiblePainted.
This sets pointer-events to painted instead, allowing pointer events even when the visibility is set to hidden

Before

You had to hover over the text first, then move the mouse to the left to click the icon.

After

Hovering to the left of the heading also shows the link icon.

[sidharthachatterjee]

Love this little but incredibly useful fix! Thanks for making gatsby-theme-blog better, Nicky! 💜