Render Global Style notice only when necessary

[okmttdhr]

Related to #84676, #79223

Proposed Changes

Currently, there's unnecessary DOM by Global Style notice even when the user is not using Global Style. This PR changes to render Global Style notice only when it is in use.

While this change appears trivial, I expect it to reduce the complexity when we add the new notice in the same place (See #84676). I'll create the follow-up PR to ensure showing only one notice simultaneously.

before	after

Testing Instructions

• Run install-plugin.sh editing-toolkit fix/gs-notice on your sandbox
• Prepare a free site
• Sandbox your site
• Go to Appearance > Editor (https:///wp-admin/site-editor.php)
• Confirm it does not render Global Style notice when Global Style is NOT in use (e.g., See document.querySelector('.wpcom-global-styles-notice-container') returns null)
• Add custom styles from the Style navigation
• Confirm it shows Global Style notice
• Confirm it sends the tracking event (calypso_global_styles_gating_notice_view_canvas_show)
• Undo your change in Global Style
• Confirm it does not render Global Style notice

On Atomic sites, we don't show the Global Style notice. You can test that breaks nothing, too, by downloading/uploading the plugin .zip file from TeamCity PCYsg-ly5-p2#atomic-testing.

Pre-merge Checklist

• Has the general commit checklist been followed? (PCYsg-hS-p2)
• https://wpcalypso.wordpress.com/devdocs/docs/testing/index.md for your changes?
• Have you tested the feature in Simple (P9HQHe-k8-p2), Atomic (P9HQHe-jW-p2), and self-hosted Jetpack sites (PCYsg-g6b-p2)?
• Have you checked for TypeScript, React or other console errors?
• Have you used memoizing on expensive computations? More info in Memoizing with create-selector and Using memoizing selectors and Our Approach to Data
• Have we added the "[Status] String Freeze" label as soon as any new strings were ready for translation (p4TIVU-5Jq-p2)?
• For changes affecting Jetpack: Have we added the "[Status] Needs Privacy Updates" label if this pull request changes what data or activity we track or use (p4TIVU-ajp-p2)?

[github-actions]

Calypso Live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-89631

Jetpack Cloud live (direct link)

https://calypso.live?image=registry.a8c.com/calypso/app:build-89631&env=jetpack

[matticbot]

This PR modifies the release build for the following Calypso Apps:

For info about this notification, see here: PCYsg-OT6-p2

• editing-toolkit

To test WordPress.com changes, run install-plugin.sh $pluginSlug fix/gs-notice on your sandbox.

[matticbot]

This PR does not affect the size of JS and CSS bundles shipped to the user's browser.

Generated by performance advisor bot at iscalypsofastyet.com.

[taipeicoder]

Tested and LGTM 👍