bulma (base): loading screen: when the `loading_screen` variable is set before extending `base/base.html`, a loading modal is shown (and must be removed via JavaScript). This avoids the user to try to use an interface which is not reactive or working normally because JS is not fully loaded yet.

{% extends 'error/base.html' %}

{% block body %}
<h1>{% trans %}Unauthorized{% endtrans %}</h1>
<p>{% trans %}Sorry, you are not authorized to access this page. An authentication is needed.{% endtrans %}</p>
{% endblock body %}