fix: alert mail display invalid url (subpath in the SENTRY_URL_PREFIX was dropped)

for example::

    >>> settings.SENTRY_URL_PREFIX='http://example.com/sentry'
    >>> url='/team/project/1/'  #generated by django.core.urlresolvers.reverse
    >>> urljoin(settings.SENTRY_URL_PREFIX, url)
    'http://example.com/team/project/1/'