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
Get TITLE and DESCRIPTION from siteconfig directly

Instead of changing django.conf.settings at runtime which we should not
be doing:
    https://docs.djangoproject.com/en/dev/topics/settings/#altering-settings-at-runtime
and is the source of bug 3150.