Bug 46027, bug 49484: Drop mw:ExtLink/URL

* Changes to the content of URL links in VE were not properly handled, as the
  VE never changed the type to mw:ExtLink on such change.

* Instead of requiring clients to implement such a type change, abstract
  syntactical detail about URL link vs. bracketed external link in Parsoid.
  Automatically switch between formats as required, but preserve non-minimal
  links of the form [http://example.com http://example.com].

* Serializer support for mw:ExtLink/URL remains, so cached HTML will continue
  to work for now. This can be dropped after cache clearing.

* The VE handles mw:ExtLink content well, and will also do so for links that
  were formerly marked as mw:ExtLink/URL. Once cached HTML is cleared the VE
  can also drop mw:ExtLink/URL handling.

Change-Id: Ibb4c27b799016c7a41efb23c03978794c1d2f012