Cleanup application related fields retrieval

Introducing new tags:
- render_app_name
- render_app_label
- render_app_description

This should replace the entries like:

{% render_with_template_if_exist app.name|lower|add:"/admin_app_name.html" app.name %}
by
{% render_name app%}

While at it use the fix improper use of app.name with template paths. The template
path should not be built using app.name as it is the verbose name that is pushed to
the template system. From https://github.com/django/django/blob/master/django/contrib/admin/sites.py:

app_dict[app_label] = {
    'name': apps.get_app_config(app_label).verbose_name,
    'app_label': app_label,
    'app_url': reverse(
        'admin:app_list',
        kwargs={'app_label': app_label},
        current_app=self.name,
    ),
    'has_module_perms': has_module_perms,
    'models': [model_dict],
}