Editor: Show 'View Post' for any post the author can read.

This expands it to private posts and matches the logic in the toolbar.

props mordauk, SergeyBiryukov.
fixes #27059.

Built from https://develop.svn.wordpress.org/trunk@27483


git-svn-id: http://core.svn.wordpress.org/trunk@27327 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Allow for custom authentication handlers for all requests.

Turn the logic used by wp_get_current_user() into a determine_current_user filter.

props rmccue.
fixes #26706.

Built from https://develop.svn.wordpress.org/trunk@27484


git-svn-id: http://core.svn.wordpress.org/trunk@27328 1a063a9b-81f0-0310-95a4-ce76da25c4cd