fix: correct the fontawesome css reference in 404 template

Change to `vendors/@fortawesome/fontawesome-free/css/all.min.css`
Previous value was based on the older `grunt` setup, and was giving the following error:
  - `Missing staticfiles manifest entry for 'vendors/fontawesome/css/all.min.css'`

{{cookiecutter.project_slug}}/{{cookiecutter.project_slug}}/templates/404.html

@@ -17,7 +17,7 @@
     <!-- Bootstrap Core CSS -->
     {% raw %}<link href="{% static 'vendors/bootstrap/css/bootstrap.min.css' %}" rel="stylesheet">
     <!-- FontAwesome CSS -->
-    <link rel="stylesheet" type="text/css" href="{% static 'vendors/fontawesome/css/all.min.css' %}">{% endraw %}
+    <link rel="stylesheet" type="text/css" href="{% static 'vendors/@fortawesome/fontawesome-free/css/all.min.css' %}">{% endraw %}
 
     <style>
     /* Error Page Inline Styles */