refactor(css): set externalIcon size in CSS

[jvoisin]

There is no need to repeat width="16" height="16" for every single externalIcon in the HTML, when we can specify this in the CSS instead.

[fguillot]

I believe it was done this way to prevent layout shift on slow connections. When the width/height attributes are defined directly in HTML, the browser knows the size before the image and stylesheet loads, so it can reserve space in the layout. If you only use CSS, the browser may not reserve the correct space until the CSS is parsed.

[fguillot]

https://developer.mozilla.org/en-US/docs/Glossary/CLS (web.dev may have a better doc)

Layout shifts may be caused, for example, by or elements that are not given width and height attributes, so the browser doesn't know how much space they will occupy until they are loaded.

[jvoisin]

Even by using the GPRS setting in the devtools of Firefox (50 Kbps download/20 Kbps upload/500ms lattency minimum), I don't notice any layout shift.