Currently, when viewing the block editor with JavaScript disabled, the user sees a blank admin page with the admin menu sidebar. This adds an admin notice informing the user that JavaScript is required for the new block editor.
Props mkaz, pento, azaozz, ocean90, desrosj.
Fixes#45453.
Built from https://develop.svn.wordpress.org/trunk@44437
git-svn-id: http://core.svn.wordpress.org/trunk@44268 1a063a9b-81f0-0310-95a4-ce76da25c4cd