OOUIHTMLForm: Support setWrapperLegend()

OOUIHTMLForm: Support setWrapperLegend()

Changed FormSpecialPage not to call setWrapperLegend() for OOUI
forms to preserve current default behavior.

Bonus:
* Correct documentation of setWrapperLegend() to state that it
  HTML-escapes legend text.
* Remove hard-coded class="visualClear" in getFormAttributes().
* Allow setWrapperLegend( true ) to display the wrapper without
  legend text.
* Rejigger things so that we can put the legend and "header HTML"
  into correct order.

Bug: T103026
Change-Id: I847c5e18ae5469aa3a68cc9fa37b2a6614476ca2