set merge options and do the actual merge of two or more topics.
the merge options screen:
* shows topics to be merged and allows to set some merge options.
* is accessed by ?action=mergetopics;sa=options.and can also internally be called by QuickModeration() (Subs-Boards.php).
* uses 'merge_extra_options' sub template of the MoveTopic template.
the actual merge:
* is accessed with ?action=mergetopics;sa=execute.
* updates the statistics to reflect the merge.
* logs the action in the moderation log.
* sends a notification is sent to all users monitoring this topic.
* redirects to ?action=mergetopics;sa=done.