Remove Fira font files

* Remove `font/` directory with Fira fonts
* Remove corresponding CSS references to Fira fonts

https://trello.com/c/abHQcg6W
Resolve Safari SVG rendering

* Remove old Middleman brand assets
* Move `middleman-logo.svg` outside of `logos/`
* Modify box-shadow value of global nav style

https://trello.com/c/YdBlvKwh