Menus: Denote the special pages in the Customizer menu editor.

The special pages here are the pages that are used for the Privacy Policy, Home, and the Posts page.

Fixes #46829.
Props garrett-eclipse, audrasjb.

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git-svn-id: https://core.svn.wordpress.org/trunk@47539 1a063a9b-81f0-0310-95a4-ce76da25c4cd