make sure @group is a Group in avatar field - fixes #4096

otherwise it would try new_network_avatar_path
validations to prevent networks from joining networks

Also blocking committees of Networks from joining other networks.

Why? Because we need to prevent endless loops.

These are very uncommon usecases anyway and there are lot's of
ways to work around them - namely making one network a group or making
one network public and open and linking it from the other one.