refer to local leaflet css and js instead of external resource in game_dashboard.html

app/templates/game_dashboard.html

@@ -2,12 +2,8 @@
 
 {% block head %}
 {{ super() }}
-<link rel="stylesheet" href="https://unpkg.com/leaflet@1.6.0/dist/leaflet.css"
-    integrity="sha512-xwE/Az9zrjBIphAcBb3F6JVqxf46+CDLwfLMHloNu6KEQCAWi6HcDUbeOfBIptF7tcCzusKFjFw2yuvEpDL9wQ=="
-    crossorigin=""/>
-<script src="https://unpkg.com/leaflet@1.6.0/dist/leaflet.js"
-    integrity="sha512-gZwIG9x3wUXg2hdXF6+rVkLF/0Vi9U8D2Ntg4Ga5I5BZpVkVxlJWbSQtXPSiUTtC0TjtGOmxa1AJPuV0CPthew=="
-    crossorigin=""></script>
+<link rel="stylesheet" href="{{ url_for('static', filename='assets/leaflet/leaflet.css') }}" />
+<script src="{{ url_for('static', filename='assets/leaflet/leaflet.js') }}"></script>
 {% endblock %}
 
 {% block app_content %}