frontend: alerter: close immediately when unlocked

[ES-Alexander]

Current behaviour

Locking keeps it open, and when the user wants to stop that they click to unlock, but the only visual change is the a slight colour shift of the tiny lock icon (which may even be under their mouse), so it's not very clear if unlocking worked.

Expected or desired behaviour

Given unlocking is already an intentional action to stop the locked open state, it's likely intuitive to just close the expanded region immediately upon unlock - if the user wants to see it again they're already close to the widget, so can just move over it again.

Prerequisites

• I have checked to make sure that a similar request has not already been filed or fixed.