Restored site header text in password reset view.

It's the one shown when the optional integration described in
https://docs.djangoproject.com/en/dev/ref/contrib/admin/#adding-a-password-reset-feature
is used.

Follow-up to commits 6f470650 and 1d42a86e, together they fix
different small UI regressions after a962286b.

Refs #21293.

django/contrib/admin/templates/admin/base_site.html

 {% extends "admin/base.html" %}
 
-{% block title %}{{ title }} | {{ site_title }}{% endblock %}
+{% block title %}{{ title }} | {{ site_title|default:_('Django site admin') }}{% endblock %}
 
 {% block branding %}
-<h1 id="site-name"><a href="{% url 'admin:index' %}">{{ site_header }}</a></h1>
+<h1 id="site-name"><a href="{% url 'admin:index' %}">{{ site_header|default:_('Django administration') }}</a></h1>
 {% endblock %}
 
 {% block nav-global %}{% endblock %}