Neuron navigator: allow requests to user-list API to ignore users

When using the POST HTTP method to request a user-list, it is now
possible to add a list of user IDs to ignore in the response. This is
needed for the user filters within the neuron navigator. There is no
change for requests made from existing code, i.e. regular GET requests
are still supported.
Neuron navigator: support user filters

When a user was selected on a user list node, it won't be shown on user
list nodes further down the path.