Code Editors: Add a persistent notification on CSS files, encouraging people...

Code Editors: Add a persistent notification on CSS files, encouraging people to use the CSS panel in the Customizer instead of editing their CSS files directly.

Props helen, westonruter, nic.bertino, michelleweber.
Fixes #42100.

Built from https://develop.svn.wordpress.org/trunk@42013


git-svn-id: https://core.svn.wordpress.org/trunk@41847 1a063a9b-81f0-0310-95a4-ce76da25c4cd