Neuron annotations: make sure queried nodes are always in same order
Neuron navigator: name a user filter only with the login
Neuron navigator: add placeholder entry for co-annotations
Neuron navigator: fix div id naming typo
Neuron navigator: show user list in jQuery Datatable

To do this, a new back-end method had to be created that responds with a
jQuery Datatable data structure and is able to react to sorting requests
from it.
Neuron navigator: remove left margin in navigator widget

The datatable looked not very well with it, so I removed it. For text
based content it needs to be added separately again.
Revert "Neuron navigator: allow requests to user-list API to ignore users"

This reverts commit 11b35aaa6a21d9cded1768aedbebd6ba616cf179. This
functionality isn't needed anymore.
Neuron navigator: make nodes add content themself

Allowing nodes to directly modify the container makes it much easier for
them to bind events to added content.
Neuron navigator: add user ID to user table node
Neuron navigator: let click on user table row create user filter node