Round trip alien extensions correctly when edited

ce.MWExtensionNode
* Build up the wikitext for round-tripping using jQuery, and pass
  through the attributes from data-mw

dm.MWExtensionNode
* Use a static getExtensionName to correctly get the extension
  name from the dataElement (as the getMatchRdfaTypes hack won't
  work for MWAlienExtensionNode's)

dm.MWAlienExtensionNode
* Implement new static getExtensionName function

Bug: 53543
Change-Id: Id4e83c14ec68c3b3970d05317477f19aaf31abe4
Merge "Round trip alien extensions correctly when edited"