Add support for unlocking users in admin settings

### What does this MR do?

This MR gives the ability to unlock a user from the Admin panel.

### Why was this MR needed?

If a user fails to login a certain number of times, Devise locks the user out for a certain amount of time (e.g. 10 minutes). There is no way for an admin to unlock a user if this happens (aside from editing the `locked_at` field entirely).

### Screenshots

#### Admin index page

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/14c661d1d1f026770997b19af714c514/image.png)

#### Admin edit user page

![image](https://gitlab.com/gitlab-org/gitlab-ce/uploads/0c6f795757e5f03eeb923d818b7d6a96/image.png)

### What are the relevant issue numbers?

This replaces !288.

Closes https://github.com/gitlabhq/gitlabhq/issues/9381

See merge request !922