[FIX] foums: WYSIWYG only works properly in forums if using wysiwyg_htmltowiki (wiki mode), so check for that on the textarea and show a warning on the admin page if using html mode

git-svn-id: file:///var/local/code.tiki.org/tikisvn/branches/13.x@52981 b456876b-0849-0410-b77d-98878d47e9d5