Add possibility to unlock an account

[jonasraoni]

Once an account is locked, it would be interesting to have a way to unlock it in an easier way than resetting the password (e.g. simple email with a link to unlock the account).

Someone with access to the plugin could probably be able to view the list of locked users with the attempts (and perhaps unlock them from there as well, if that's a valid use case).

Also, it might make sense to inform for how long the account will be locked and share instructions about how to unlock it (e.g. reset the password).