Prevent deleting group used in locations

[jakub-tldr]

Currently, we don't allow using empty group list as Allowed groups in Location settings.

However, deleting group that is the only group used in Location bypass this, making location inaccessible to non-admin users.

Possible solution:

• Show a warning before deletion listing affected locations. If proceeding would leave any location with an empty group list, block and require the user to fix those locations manually first.