Admin Bar: Make it consistent between Calypso and WP Admin regardless of the value of Admin Interface Style (take two)

[fushar]

Take two of #38362
Fixes https://github.com/Automattic/dotcom-forge/issues/8314
Fixes https://github.com/Automattic/dotcom-forge/issues/8311

Proposed changes:

• We want to make the admin bar consistent everywhere, so this PR adds the flag WPCOM_ADMIN_BAR_UNIFICATION to determine whether to display the consistent admin bar. If the flag is true, we should not initialize the custom Masterbar and avoid the CSS concat for the color schemes due to the loading CSS priority.

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?
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Jetpack product discussion

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

Testing instructions:

• Apply changes to your site, either atomic or simple
• Apply D155601-code to your simple site
• Check the admin bar on both WP Admin and Calypso
• Change the value of the Admin Interface Style
• The admin bar should be always the same

[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 feat/unify-masterbar-2 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 feat/unify-masterbar-2
  

  bin/jetpack-downloader test jetpack-mu-wpcom-plugin feat/unify-masterbar-2
  

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[github-actions]

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Mu Wpcom plugin:

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