Bug 53432: DOM spec cleanup

* mw:WikiLink/Interwiki becomes mw:ExtLink. We automatically serialize
  links matching an interwiki URL as wiki links.
* mw:ExtLink/* all become just mw:ExtLink. We transparently handle the
  serialization to special ISBN/RFC/PMID and auto-numbered forms.
  Auto-numbered external links now have an empty content, which will be
  auto-numbered with CSS as documented in bug 53505.
* mw:WikiLink/Category becomes mw:PageProp/Category
* mw:WikiLink/Language becomes mw:PageProp/Language

Change-Id: I0f0a826c6a56e9d74d92fefdc29dc2503e7908e3