Add 404 page

theme/templates/404.html

+{% extends "base.html" %}
+{% comment %}
+SPDX-License-Identifier: GPL-3.0-or-later
+{% endcomment %}
+
+{% load i18n static %}
+
+{% block content_title %}
+    <h1>Oh crap! <small>404</small></h1>
+{% endblock %}
+
+{% block content %}
+    <p>
+        {% trans "This page has been lost in the deep dark." %}
+    </p>
+    <p>
+        {% blocktrans %}
+            If you think this page sould exist, please
+            <a href="mailto:club-med@crans.org">alert the administrator</a>.
+            Else, you should go back and blame your link.
+        {% endblocktrans %}
+    </p>
+    <img src="{% static "images/butterflies.svg" %}" class="poulpy" alt="erdnaxe's butterflies! Designed by Freepik">
+{% endblock %}