Mark reference preview content area as "mw-parser-output"
This CSS class is used by the regular wikitext content area, and used
by certain CSS selectors (e.g. the external web link arrow).
The DOM structure for each popup is (intentionally) created outside
of this scope, at the end of the current page's body. This works
great for page previews because the do *not* want to share any styles
with the rest of the page. But reference previews want to do this.
In this patch I also remove the inner <span>. It was misplaced (note
the name) and resulted in block elements nested in an inline element.