add integrity hash for Font Awesome script (#1922)

* add integrity hash for Font Awesome script

As the script is loaded from an external CDN (use.fontawesome.com), its integrity must be ensured mainly for security reasons.
This hash must be updated together with the exact version of Font Awesome, as given on their website.

* Update to FontAwesome 5.5.0

_includes/scripts.html

@@ -9,7 +9,7 @@
   {% endfor %}
 {% else %}
   <script src="{{ '/assets/js/main.min.js' | relative_url }}"></script>
-  <script src="https://use.fontawesome.com/releases/v5.3.1/js/all.js"></script>
+  <script src="https://use.fontawesome.com/releases/v5.5.0/css/all.css" integrity="sha384-B4dIYHKNBt8Bc12p+WXckhzcICo0wtJAoU8YZTY5qE0Id1GSseTk6S+L3BlXeVIU"></script>
 {% endif %}
 
 {% if site.search == true or page.layout == "search" %}