Only shown if VisualEditorEnableDiffPage is set, or query
string param 'visualdiff' is present.
Currently:
* All VE javascript is loaded (could be cut down to just DM code)
* The entire Parsoid HTML of both revisions being compared is loaded
* Both Parsoid HTML docs are parsed into VE DM trees and diffed
Bug: T167508
Change-Id: I151fc9bab3d3032f50c8d11be6b54e45a06fcc34
