Add missing twig template for authsource list in old core module

Closes: #1217

modules/core/templates/authsource_list.twig

+{% set pagetitle = 'Test Authentication Sources'|trans %}
+{% extends "base.twig" %}
+
+{% block content %}
+    <h2>{{ pagetitle }} </h2>
+    <ul>
+    {% for key, name in sources %}
+        <li><a href="?as={{ name|escape('url') }}">{{ name }}</a></li>
+    {% endfor %}
+    </ul>
+{% endblock %}