Create a custom search index for the messages table.
Called by ?action=admin;area=managesearch;sa=createmsgindex.
Linked from the EditSearchMethod screen.
Requires the admin_forum permission.
Depending on the size of the message table, the process is divided in steps.
uses ManageSearch template, 'create_index', 'create_index_progress', and 'create_index_done'
sub-templates.