Add REGEXP to meta queries.

"REGEXP is usually a terrible idea to use, but I see no reason to disallow it."

props wonderboymusic.
fixes #18736.

Built from https://develop.svn.wordpress.org/trunk@25525


git-svn-id: http://core.svn.wordpress.org/trunk@25445 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Make it possible to filter comment screen by post type; no UI. props johnbillion, c3mdigital. fixes #18596.

Built from https://develop.svn.wordpress.org/trunk@25526


git-svn-id: http://core.svn.wordpress.org/trunk@25446 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Allow the "Add New" URL on post edit screens to be filtered via `admin_url()`. props SergeyBiryukov, jeremyfelt, c3mdigital. fixes #18504.

Built from https://develop.svn.wordpress.org/trunk@25527


git-svn-id: http://core.svn.wordpress.org/trunk@25447 1a063a9b-81f0-0310-95a4-ce76da25c4cd