Self-host FontAwesome assets

[jducoeur]

We needed to update FA, in order to get the Mastodon logo. The quickest and easiest way way to set up a kit on their CDN, and use that.

That works fine, but is inappropriate from an archival POV; we shouldn't be depending on an external CDN if we can avoid it. So at some point, we should redo the way we are loading the FA assets, to self-host them in our website instead.