Make default group configurable regarding access rights and internal connections

[sebvonhelsinki]

We have the use case, that we want to exclude certain users from accessing the devices in the default group, and we want to prohibit connections between devices in the default group. Both are settings that can be configured for other groups, but not for the default group.

We already enabled the "only admins can access unassigned devices" checkbox, but we need more detailed control since we have non-administrators that still should only be able to access devices in their own group, and not any new device that gets added.