(Bug 51944) Tweaks to wikitext escaping of [[ and ]] in link text

* Fixed bug in wikilink wt-escaping handler.
* Updated existing tests to reflect this + other recent changes
  to wikilink parsing which leads to a previously failing html2wt
  test to go green.

Change-Id: I53ed04c054765211089a1fe53fde4c0d668186f7