Customize: Allow notifications for linting errors in code editor control (for...

Customize: Allow notifications for linting errors in code editor control (for Additional CSS) to be overridden to allow saving.

Implements the same override that was implemented in [41721] for the theme/plugin editors.

See #41897, #41887.
Fixes #42528.

Built from https://develop.svn.wordpress.org/trunk@42171


git-svn-id: https://core.svn.wordpress.org/trunk@42001 1a063a9b-81f0-0310-95a4-ce76da25c4cd