include fontawesome instead of using cdn

Author: Jannik-Hm

_includes/head-custom.html

@@ -0,0 +1,5 @@
+<link href="assets/css/fontawesome.css" rel="stylesheet">
+
+{% if site.favicon %}
+  <link rel="shortcut icon" type="image/x-icon" href="{{ '/favicon.ico' | relative_url }}">
+{% endif %}