(Bug 62341) WTS fix to use regular wikilinks if nowikis show up.

* Dont wikitext escape the link target in simple-wikilink serialization
  scenarios since that modifies the link target (either generates an
  invalid wikilink target or a different target). But, entities are
  always escaped.

  For valid page names that have a wikitext representation, this ensures
  that they remain valid.

  For invalid page names that don't have a wikitext representation,
  this will change the page name to point to something that may or
  may not exist. For now, I am leaving this unchanged since VE itself
  doesn't provide hrefs that cannot be produced by a valid wikipage.

  The valid/invalid href detection relies on the same test that is used
  by the parse pipeline to render invalid wikilinks to text.

* Added a new parser test that verifies that wikilink hrefs are not
  subjected to nowiki escaping tests.

Change-Id: Ie661a586bb050171811db8d975c91e1a286b7aa3