Add custom base template for admin site

This is necessary since after gathering all the templates in just one place
the admin/base.html template is being used to render the default Django admin
site located in /django_admin/ and the default look is more suitable.

This is just a merge of default templates for admin site from Django:

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