FIX Ensure site tree expands to show current child

[GuySartorelli]

There are various bugs related to expanding the site tree when the current item isn't loaded into the tree, which whis PR should fix.

Fixed scenarios

• If the currently loaded page is one level deeper in the tree than what is currently loaded, the node will be loaded from the server and added to the tree but the tree won't expand.
  This PR fixes it by expanding the relevant parent after the child node is added to the tree.
• If the currently loaded page is at least two levels deeper in the tree than what is currently loaded, self.jstree('load_node', -1); is called but results in an error (the main problem the linked issue is about)
  This PR fixes it by reinstating the mandatory callback passed to load_node which was removed in FIX jstree state when saving a page by retaining the open/closed state and selected node state #932

Remaining scenarios

• If the currently loaded page is at least three levels deeper in the tree than what is currently loaded, self.jstree('load_node', -1, callback); is called but nothing happens. Looks like that's not recursive.

Issue

• CMS > Pages > jsTree collapses with console error for large sites silverstripe-cms#3144