Allow admins to unlock users

Created by: mrnugget

If a user is locked out because they ran into too many attempts, there's no way for an admin to unlock them.

That's especially a problem if email hasn't been setup on the instance and the user didn't receive a "here's how to unlock yourself" email.

I propose we add a "Unlock" button to the site-admin area in case a user is locked out that then calls this: https://github.com/sourcegraph/sourcegraph/blob/9d9e7264cfde7bc25e0489a93d90320018aad8dd/cmd/frontend/internal/auth/userpasswd/lockout.go#L173-L178

/cc @sourcegraph/iam