Add "Confirm user" button to admin page

### What does this MR do?

This MR adds a "Confirm user" to the admin user page.

### Why was this MR needed?

There is no way for an admin to force a user confirmation. Some users create users in the API and then don't have a way to force the user into a confirmed state.

Originally, I attempted to add a link to allow the admin to resend a confirmation e-mail. But that may not be helpful because a user can be in the unconfirmed state with no `unconfirmed_email` set.

### Screenshot

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0571b1e27597289b760a72948ba869e5/image.png)

### What are the relevant issue numbers?

* Closes #2116
* Closes https://github.com/gitlabhq/gitlabhq/issues/9502

See merge request !1067