Allow Admin to filter users by 2FA status

> ![Screen_Shot_2015-06-19_at_4.38.12_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/deba7f2a6b8d1548c1d1ac401e0e35a1/Screen_Shot_2015-06-19_at_4.38.12_PM.png)

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2414

See merge request !852