Make it possible to find / filter on confidential issues only

Related: https://gitlab.com/gitlab-org/gitlab-ce/issues/15288 From: gitlab-com/www-gitlab-com!2061 (comment 11655440)

This would be of use as more confidential issues are created, that cannot always be found easily by use of labels, e.g. when created by a non-project member (also see gitlab-com/www-gitlab-com!2061 (comment 11656570) in the same discussion as above).