This is a bit of "one step forward, two steps back" in that we are
now using the much more lean SVG icons—and only the ones we are ac
tually using—instead of having to download the two ~70KiB web font
files, but it means we need to use JavaScript.
This approache was inspired by the work @xekon did in #127.
See: https://fontawesome.com/how-to-use/on-the-web/advanced/svg-javascript-core
See: https://github.com/alanorth/hugo-theme-bootstrap4-blog/pull/127
