fix wrong link in email caused by duplicate path prefix

If sentry is deployed under URL prefix http://xxx.com/sentry, the link is email will be 'http://xxx.com/sentry/sentry/(team)/....'

Because the link is concat by 'settings.URL_PREFIX' and reverse(). Obviously, django's reverse() will return relative url containing path prefix like '/sentry/(team)/...' . And 'settings.URL_PREFIX' will contain "/sentry" too.