Creates a custom search index for the messages table.

• Called by ?action=managesearch;sa=createmsgindex in 1.1 and ?action=admin;area=managesearch;sa=createmsgindex in 2.0
• Linked from the EditSearchMethod screen.
• Requires the admin_forum permission.
• Uses the 'create_index', 'create_index_progress', and 'create_index_done' sub templates of the ManageSearch template.
• Depending on the size of the messages table, the process is divided into steps (to prevent timeouts and overloading the server).