Fix missing "Theme Showcase" sub menu item on Simple Classic site Calyspo pages

[DustyReagan]

Fixes https://github.com/Automattic/dotcom-forge/issues/8521

Proposed changes:

• This PR fixes a bug so the "Theme Showcase" menu appears on Simple Classic site Calypso pages under the "Appearance" menu.
• In this PR we move the "Classic site gate" inside the hook callback function. This fixes the issue because Calypso gets the menu data via an API call. That API call user does not have the "Classic site option" enabled. Eventually, the API call switches to the calling user. At that point, we can accurately check for the wpcom_admin_interface site option. By the time the admin_menu action is called, the API has switched to the correct user, and things work as expected.

Before	After

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:

• Load this PR on your sandbox using the instructions in the comment below.
• Go to a Simple classic site Calypso page like /plans/[site] and view the "Theme Showcase" under the "Appearance" menu.
• Go to /wp-admin/themes.php and ensure the banner still displays on Classic sites.

[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 WordPress.com Simple 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, and enable the fix/simple-theme-showcase-menu branch.

  • For jetpack-mu-wpcom changes, also add define( 'JETPACK_MU_WPCOM_LOAD_VIA_BETA_PLUGIN', true ); to your wp-config.php file.

• To test on Simple, run the following command on your sandbox:

  bin/jetpack-downloader test jetpack-mu-wpcom-plugin fix/simple-theme-showcase-menu
  

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]

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[fushar]

I see... innocent bug. I think I broke it on #38364 🤦‍♂️

This fixes the issue for me, thanks!