Sets merge options and does the actual merging of two or more topics.

• The merge options screen:
• Shows topics to be merged and allows the user 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 SplitTopics 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 to all users monitoring this topic.
• Redirects to ?action=mergetopics;sa=done.
• Same as MergeTopics2 in 1.0.x