Facia tool: native drag 'n' drop

* Drag from a other sites directly into collections.
* Remove jQueryUI (hundreds of files) and bootstrap.js.

NB. Still needs some extra validation against ContentAPI, for dragged-in content.

Relavant js file is:
https://github.com/guardian/frontend/blob/8235a34eb45549e85dda7ae2ada301b116240bef/facia-tool/public/javascripts/models/droppable.js