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

Admin edit user page

What are the relevant issue numbers?

This replaces !288 (closed).

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