Admin override for Screen Time

We should allow logging into an account that would otherwise be limited by Screen Time & Limits with admin approval.

An example use case is that you set up Screen Time for your kid to not allow computer usage after 8 in the evening, but they need to log in to work on a project that night before it is due. It seems like they should be able to log back in with an admin's approval even outside of their regular Screen Time.

It would also help resolve this case: https://github.com/elementary/switchboard-plug-parental-controls/issues/126

An open question is how long that approval should last? This might also need coordination with the screen time & limits daemon itself.