[ticket/12270] Correct notifications for posts/topics now working

This patch should finally work correctly: Add the correct
notification for post/topic approval without touching templates.

This has been done by calling the function approve_topics()
inside the approve_posts() function. It's the simplest way for
this to work correctly. There may be a call to the DB but other
ways (if any), without touching templates,  would have been too
messy for a function like this.

PHPBB3-12270