Redirect to a default path if HTTP_REFERER is not set

Safari 9.0 does not yet honor the HTML5 `origin-when-cross-origin` mode,
and it's possible load balancers/proxies strip the HTTP_REFERER from
the request header. In these cases, default to some default path.

Closes #3122

Closes https://github.com/gitlabhq/gitlabhq/issues/9731

See merge request !1646