Site Editor: Add get parameter to hide admin bar in classic theme site preview

[t-hamano]

Trac ticket: https://core.trac.wordpress.org/ticket/63076

This PR hides the admin bar if the get parameter wp_site_preview=1 is present.

This PR is needed to hide the admin bar from the classic theme site preview in the site editor.

This PR cannot yet be tested in the site editor, but can be tested on the frontend.

• Admin bar is not visible:
  • http://localhost:8889/?wp_site_preview=1
• Admin bar is visible:
  • http://localhost:8889/
  • http://localhost:8889/?wp_site_preview=2

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[github-actions]

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[t-hamano]

I'm wondering whether the JS code that disables interactive elements should also be moved to this PR and output as an inline JS.

[github-actions]

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[stokesman]

I'm wondering whether the JS code that disables interactive elements should also be moved to this PR and output as an inline JS.

I think it’s a good idea because otherwise the elements can be interactive for a moment before the load event. Yet I also think we may want the flexibility to modify that logic without having to change core code. If so, we might make the inline JS merely post a message that the parent window can listen for.