Allow customizing dismiss popup view

I need to give some background info to this pull request. I use the django admin with a modified ``document.domain`` (``example.com`` instead of ``www.example.com``). So the popup and the main window are in a different domain which makes the ``opener.dismissPopupAndReload`` call resulting in a browser cross-domain permission error.

I am fixing this by extending the ``base_site.html`` which is modifying the ``document.domain`` setting admin-wide.