chat/message: fix urlize escaping on nunjucks: Nunjucks imply to use `safe` when using `urlize`, while Jinja2 doesn't. Esacaping and safe filter are added so that `urlize` can work with both engines.

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

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