Invalid URLs in mails if sentry is not available on /

Hi,

I load sentry URL as following.

``` python
(r'^sentry/', include('sentry.web.urls'))
```

I also set SENTRY_URL_PREFIX to http://foobar.com/sentry

When I receive a mail, the URL is http://foobar.com/sentry/sentry/1/group/840/ but should be http://foobar.com/sentry/1/group/840/

If I set SENTRY_URL_PREFIX to http://foobar.com then api calls can't find their ways.

Thanks!
