WP_Screen: Whitelist -new and -add screens, and prevent edit-comments and...

WP_Screen: Whitelist -new and -add screens, and prevent edit-comments and edit-tags from being mashed on. fixes #19353.

git-svn-id: https://core.svn.wordpress.org/trunk@19471 1a063a9b-81f0-0310-95a4-ce76da25c4cd