xpost redirects should be 301's rather than 302

As reported in https://meta.trac.wordpress.org/ticket/4775 the xpost redirects should 301 (Permanent) to their respective locations rather than just 302'ing (Temporary)

Code ref: https://github.com/Automattic/o2/blob/master/inc/xposts.php#L572-L588

Start in inc/xposts.php at lines 572-588 and inspect how xpost redirects are created. Change the redirects from temporary to permanent, then verify that each xpost redirects to its respective location with a 301 status.

Written by the indexing model from the issue text.