Appearance menu: Add Additional CSS menu for classic themes

[BogdanUngureanu]

This PR re-adds the Additional CSS submenu item that was removed from Jetpack. The menu is visible only for Atomic sites that are using a classic theme.

Fixes linear DOTCOM-10681

Proposed changes:

• Bring back Additional CSS in Appearance menu for Atomic sites

Other information:

• Have you written new tests for your changes, if applicable?
• Have you checked the E2E test CI results, and verified that your changes do not break them?
• Have you tested your changes on WordPress.com, if applicable (if so, you'll see a generated comment below with a script to run)?

Jetpack product discussion

Does this pull request change what data or activity we track or use?

Testing instructions:

• Use Jetpack beta and install this branch on your AT Dev Site
• Notice that the Additional CSS submenu is now visible in Appearance
• Click on it
• Notice that you are redirected to the Additional CSS page
• Click X (close)
• Notice you are redirected back to the correct page on your site

[github-actions]

Are you an Automattician? Please test your changes on all WordPress.com environments to help mitigate accidental explosions.

• To test on WoA, go to the Plugins menu on a WoA dev site. Click on the "Upload" button and follow the upgrade flow to be able to upload, install, and activate the Jetpack Beta plugin. Once the plugin is active, go to Jetpack > Jetpack Beta, select your plugin (Jetpack or WordPress.com Site Helper), and enable the fix/add-additional-css-on-atomic branch.
• To test on Simple, run the following command on your sandbox:

bin/jetpack-downloader test jetpack fix/add-additional-css-on-atomic

bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/add-additional-css-on-atomic

Interested in more tips and information?

• In your local development environment, use the jetpack rsync command to sync your changes to a WoA dev blog.
• Read more about our development workflow here: PCYsg-eg0-p2
• Figure out when your changes will be shipped to customers here: PCYsg-eg5-p2

[github-actions]

Thank you for your PR!

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[jp-launch-control]

Code Coverage Summary

Coverage changed in 1 file.

File	Coverage	Δ%	Δ Uncovered
projects/packages/masterbar/src/admin-menu/class-atomic-admin-menu.php	75/162 (46.30%)	-2.09%	7 💔

Full summary · PHP report · JS report

_{Coverage check overridden by I don't care about code coverage for this PR Use this label to ignore the check for insufficient code coveage. .}

[BogdanUngureanu]

@heavyweight yes, that's expected because we don't want to alter the Core experience. On Simple Sites we also don't display the Additional CSS submenu.