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