Fix for #3295 (preserving empty lines in wiki code blocks). The approach chosen is a workaround until the wiki formatting engine has been updated to emit markup event streams instead of big blobs of text. It requires adding an `xml:space="preserve"` attribute to the containing element of any `wiki_to_html()` expression in the templates. These attributes do make it through to the output, but that shouldn't do any harm. They'd go away if we simply serialized to HTML.