Update to Font Awesome 6

[MinchinWeb]

• [ ] decide on JS + SVG or CSS + webfont approach
  • SVG appears to be faster
  • SVG has special notes on working with jQuery (which is used by Bootstrap)
  • CSS is closer to the current setup
  • CSS allows directly including is a LESS or SCSS stylesheet -- https://fontawesome.com/how-to-use/web-fonts-with-css#less-and-sass
• [ ] replace linked files in theme
• [ ] figure out if social icons are still working (and which ones need to be "square")
• [ ] write upgrade notes (renamed icons, in particular)
• [ ] major release

https://fontawesome.com/how-to-use/upgrading-from-4

[MinchinWeb]

Upgrade straight to v6 --> https://fontawesome.com/docs/web/setup/upgrade/upgrade-from-v4