fix static asset path in base template

ckanext/mistral/templates/base.html

@@ -3,6 +3,6 @@ using Webassets. 'mistral/' is the name that the mistral/webassets directory was
 registered with when the toolkit.add_resource() function was called. 'mistral'
 is the name to the Webassets bundle file, registered inside webassets.yml file.
 #} {# % asset 'mistral/mistral' % #}
-<link rel="stylesheet" href="/styles/mistral.css" />
+<link rel="stylesheet" href="{{ h.url_for_static('/styles/mistral.css') }}" />
 
 {% endblock %}