[ticket/12304] Add CSS class to rules-link container

When a forum has a forum-rules link instead of forum-rules text, there
is no way to distinguish in the styling of the container (or the button)
it will have.

It should be possible to style the \<div class="rules"> container
differently if there is only a link (no text).

http://tracker.phpbb.com/browse/PHPBB3-12304