Generalize polkit checks from firewall

[martinpitt]

The firewall page is the first page which does a polkit check for a read-only view of the UI when the logged-in user doesn't have sufficient permissions.

A lot of other pages could use something like this as well. Choose another page where we can use this and move the functionality into pkg/lib

[ancorgs]

The check in the firewall page is certainly a step in the right direction, but please improve it before replicating the model. It introduces a hard dependency on polkit and is not fully precise, I fear. See #17669