Loads font-awesome asynchronously (mmistakes#2967)

Loading font-awesome asynchronously allows to display the site faster. This change is advised by google pagespeed insights

_includes/head.html

@@ -15,7 +15,8 @@
 
 <!-- For all browsers -->
 <link rel="stylesheet" href="{{ '/assets/css/main.css' | relative_url }}">
-<link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@5/css/all.min.css">
+<link rel="preload" href="https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@5/css/all.min.css" as="style" onload="this.onload=null;this.rel='stylesheet'">
+<noscript><link rel="stylesheet" href="https://cdn.jsdelivr.net/npm/@fortawesome/fontawesome-free@5/css/all.min.css"></noscript>
 
 <!--[if IE]>
   <style>
@@ -36,4 +37,4 @@
   {% for script in site.head_scripts %}
     <script src="{{ script | relative_url }}"></script>
   {% endfor %}
-{% endif %}
+{% endif %}