Fix layout of notices in posts

http://tracker.phpbb.com/browse/PHPBB3-11782

Instead of using p.rules use p.post-notice with additional class name to indicate background image.

Background is applied via :before pseudo element instead of aligning it to few pixels off paragraph's left edge to make it possible to align background for RTL languages.

Degradation: :before pseudo element is not supported by IE7. IE7 users (who's computers should melt in fire for greater good) will see notices but without icon.