fix deprecation warning about django.conf.urls.defaults

This is changing to django.conf.urls in Django v1.5.
settings.py: fix warning about mail_admins filters

It is now required to define a `filters' value in the
`mail_admins' in settings.py for Django v1.5.
extra/models.py: remove deprecated verify_exists

The `verify_exists' option has been removed in Django v1.5.
templates: fix url tag syntax for Django v1.5

The built-in template `url' tag syntax has changed in
Django v1.5, the URL name now needs to be a string.