Neuron navigator: only add users link if there was none before

If there has been a user filter before, it wouldn't make no sense if one
could add another one. At least in its current interpretation a neuron
filter restricts the result set in who owns a neuron. And since it
can't be owned by multiple users, having the filter once is enough.
Neuron navigator: add neuron list node

This node displays a list of neurons that match the filters added
further up in the path to the root. It issues a request to the neuron
query backend and prints out the information it gets.