Edit the search method and search index used.

• Called by ?action=managesearch;sa=method.
• Requires the admin_forum permission.
• Uses the 'select_search_method' sub template of the ManageSearch template.
• Allows the user to create and delete a fulltext index on the messages table.
• Allows the user to delete a custom index (that CreateMessageIndex() created).
• Calculates the size of the current search indexes in use.