Use Font Awesome SVGs via JavaScript

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

source/js/fontawesome.js

@@ -0,0 +1,13 @@
+import { library, dom } from '@fortawesome/fontawesome-svg-core'
+import { faFolder, faTag } from '@fortawesome/free-solid-svg-icons'
+import { faFacebookF, faTwitter, faLinkedinIn } from '@fortawesome/free-brands-svg-icons'
+
+// Add solid icons to our library
+library.add(faFolder, faTag)
+
+// Add brand icons to our library
+library.add(faFacebookF, faTwitter, faLinkedinIn)
+
+// Replace any existing <i> tags with <svg> and set up a MutationObserver to
+// continue doing this as the DOM changes.
+dom.watch()