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()
Beginning of copy/paste functionality

- Added Clipboard.schema
- Added addToClipboard()
Implementing basic copy/paste UI elements
Added stub for pasteFromClipboard()

Also added missing _courseId property to addToClipboard()
On-going work to clipboard discovery

This code is quite volatile right now. The client-side addToClipboard()
function may eventually hook into a server-side middleware function.
Updated clipboard storage method
Addded Clipboard collection to Origin.editor

Collection is also removed when 'Copy' is invoked.
WIP - Paste functionality
Functional paste added
- Recursive create children function
Updated addToClipboard() to make it more generic

Also removed 'referencesId' column from Clipboard.schema as this is no
longer required.
Merge branch 'block_crud' of https://github.com/adaptlearning/adapt_authoring into editor-view-refactor

# By brian-learningpool (6) and Kevin Corry (4)
# Via brian-learningpool
* 'block_crud' of https://github.com/adaptlearning/adapt_authoring:
  Updated addToClipboard() to make it more generic
  Functional paste added - Recursive create children function
  WIP - Paste functionality
  Addded Clipboard collection to Origin.editor
  Updated clipboard storage method
  Added stub for pasteFromClipboard()
  Implementing basic copy/paste UI elements
  Beginning of copy/paste functionality
  Basic article and block move functionality added
  On-going work to front-end

Conflicts:
	frontend/src/core/editor/models/editorArticleModel.js
	frontend/src/core/editor/models/editorModel.js
	frontend/src/core/editor/views/editorArticleEditView.js
	frontend/src/core/editor/views/editorArticleView.js
	frontend/src/core/editor/views/editorBlockEditView.js
	frontend/src/core/editor/views/editorPageOverviewView.js
	frontend/src/core/editor/views/editorSidebarView.js
	frontend/src/core/editor/views/editorView.js