Nodes#tree - alter UI if node doesn't have children

If the node we just loaded doesn't have any children nodes, remove the
expand/collapse triangle icon from the parent.
Tree node rendering - always asume there will be children

Instead of checking for each node if it has any children (which requires
eager load of the entire tree) to make design decission, asume they have
children and let the Nodes#tree call sort out the UI if it turns out
they don't have any.
Move success logic from jQuery.Treenav to Nodes#tree.js
Merge pull request #2 from etdsoft/ajax-nodes-tweaks

Ajax nodes tweaks