Check via Polkit if the current user is actually allowed to enroll
devices before trying to do so. If not, show a notification that
explains that a system administrator needs to authorize the device.
Clicking on the notification will guide the user to the thunderbolt
control center panel. Before this patch, when the current user was
not allowed to enroll a device a polkit dialog would pop up which
is confusing because it did not contain any information why it was
shown. This patch implements the behavior as designed (see [1],
section "Multi-user environments").
[1] https://wiki.gnome.org/Design/Whiteboards/ThunderboltAccess
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