Show user 2FA status on Admin::Users#show

| Enabled | Disabled |
| ------- | -------- |
| ![Screen_Shot_2015-06-19_at_2.58.42_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/b81f6a992366105c14e03852385b28f0/Screen_Shot_2015-06-19_at_2.58.42_PM.png) | ![Screen_Shot_2015-06-19_at_2.58.30_PM](https://gitlab.com/gitlab-org/gitlab-ce/uploads/7bb8fd11b6c27615eef19154ffabe2de/Screen_Shot_2015-06-19_at_2.58.30_PM.png) |

Closes internal https://dev.gitlab.org/gitlab/gitlabhq/issues/2415

See merge request !851