improve layout file

lib/views/layout/admin.html

@@ -1,6 +1,6 @@
 {% extends '2column.html' %}
 
-{% block html_head %}
+{% block html_additional_headers %}
   {% parent %}
   <script src="{{ webpack_asset('legacy-admin').js }}" defer></script>
 {% endblock %}

lib/views/layout/layout.html

@@ -39,6 +39,8 @@
   <link href='//fonts.googleapis.com/css?family=Open+Sans:400,700' rel='stylesheet' type='text/css'>
   <!-- Font Awesome -->
   <link href='//cdn.jsdelivr.net/fontawesome/4.7.0/css/font-awesome.min.css' rel='stylesheet' type='text/css'>
+
+  {% block html_additional_headers %}{% endblock %}
 </head>
 {% endblock %}