FU r83909: restore preprocessing stage to cache link existence using LinkBatch; other...

FU r83909: restore preprocessing stage to cache link existence using LinkBatch; otherwise a separate DB query is done on every link.  Since the ipb_by_text field is no longer in use we still have to do a cross-cast to get the usernames from the ids stored in ipb_by, but we can do that with one query via a UserArray, so it's not significantly worse than before.