auth_saml: add missing mellon base template (#33743)

src/authentic2_auth_saml/templates/mellon/base.html

@@ -0,0 +1,12 @@
+{% extends "authentic2/base.html" %}
+
+{% block extrascripts %}
+  {{ block.super }}
+  {% block mellon_extra_scripts %}
+  {% endblock %}
+{% endblock %}
+
+{% block content %}
+  {% block mellon_content %}
+  {% endblock %}
+{% endblock %}