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.

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Make it possible to filter comment screen by post type; no UI. props johnbillion, c3mdigital. fixes #18596.

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Allow the "Add New" URL on post edit screens to be filtered via `admin_url()`. props SergeyBiryukov, jeremyfelt, c3mdigital. fixes #18504.

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Revert [25031] and [24841] - filling the internal linking search field with selected text has UX and performance issues with no resolution in sight. see #16276.

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