REST API: Enable users with read_private_posts to query for them.

An authorized request with the read_private_posts capability for a post type should be able to GET /wp/v2/posts for posts of status=private. This query is further sanity-checked by WP_REST_Posts_Controller->check_read_permission(), which is unchanged.

Props rachelbaker, soulseekah, twoelevenjay.

Moves [43694] from the 5.0 branch to trunk.

Fixes #43701.
Built from https://develop.svn.wordpress.org/trunk@43979


git-svn-id: https://core.svn.wordpress.org/trunk@43811 1a063a9b-81f0-0310-95a4-ce76da25c4cd