* Will be replaced with http://wordpress.org/extend/plugins/atom-publishing-protocol/.
* Introduces an action, xmlrpc_rsd_apis, to add APIs to xmlrpc.php?rsd.
* Introduces support for 'error' being 403 and 50x in class-wp.php.
* Removes 'Remote Publishing' from Writing Settings (see [21804]). Keeps the remote_publishing settings section.
DB version is bumped to generate the new wp-app rewrite rule and remove the old enable_app option.
props wonderboymusic.
fixes#21509.
git-svn-id: https://develop.svn.wordpress.org/trunk@21818 602fd350-edb4-49c9-b593-d223f7449a82
