[ticket/11342] Mark active topics read when marking forums read

Marking forums read while viewing a category will also mark the active
topics inside that category read. Therefore, the active topics should also
be marked read. With this patch the ajax callback "mark_forums_read" will
check if the active topics id ("active_topics") exists and also mark
topics read if it does. As the active topics will only be displayed in
categories, which can't have any other topics, this is a proper approach.

Ticket: http://tracker.phpbb.com/browse/PHPBB3-11342

PHPBB3-11342