On-going work to front-end

- Implemented placeholders for cascading delete
- Page list now refreshes when a page is removed
- Implemented change suggested by Rob to get 'pre' remove working in
  Mongoose
Basic article and block move functionality added

A first version which allows moving by setting the _parentId to one of a
list of possible parents.  In future this will most likely be replaced by
drag and drop functionality.

I've also added the following helper methods to EditorModel:
- getSiblings()
- getPossibleAncestors()