Remove the experiment option for Block Theme Previews

[okmttdhr]

What?

Remove the flag that was hiding Block Theme Previews behind the experiment option, as we addressed issues in this comment #50030 (comment).

Related PRs: #50030, #50863

Why?

Since I assume there's no blocker to remove the flag. Let me know if we have something! 🙏

I refer to the following issues/feedback that we could work on them to improve the experience after this PR;

☂️

• Theme Preview experiment: Can't see how Style variations affect my existing page #50692
• Block Theme Previews: simplify Site View text or consider splitting out theme name #50920
• Block Theme Previews: show child theme information when relevant #50921
• Block Theme Previews: not able to use the Site Editor preview in "Add New" flow #50749
• Block Theme Previews: add option to switch between different themes to preview #50918
• Block Theme Previews: hide "View" when in the Edit View #50919
  • Hide the "View" button when previewing a theme #50986
• Block Theme Previews: Persist the theme_preview parameter on the front end of the site. #50713
• Block Theme Previews: Add the theme_preview parameter to the preview link in the Site Editor #50712
• First pass at themes endpoint. #41836
• Block Theme Previews: Resolve "Quirks Mode" issue #51017

How?

• Removed a setting field for gutenberg-theme-previews
• Removed the condition for the experiment

We will need to stop adding the Preview button when https://core.trac.wordpress.org/ticket/58190 lands, as it's written in https://github.com/WordPress/gutenberg/blob/release/15.8/lib/compat/wordpress-6.3/theme-previews.php#L56

Testing Instructions

To see Block Theme Previews are working;

• Go to /wp-admin/themes.php
• Click the Live Preview button on a theme installed

or

• Open the Site Editor and add the following to your URL: ?theme_preview=[themePath] where themePath is the relative path to the theme you want to preview (e.g. twentytwentythree).

To see that there's no Block Theme Previews option;

• Go to /wp-admin/admin.php?page=gutenberg-experiments

Screenshots or screencast

[ajlende]

LGTM 👍

Nothing under the ☂️ seems like a deal breaker to me, but I'll hold off from clicking the merge button in case someone else has a different opinion.

[okmttdhr]

Thanks for your review!
I marked this PR as a draft since the GET variable theme_preview is something widely used in the WordPress context, and some 3rd parties may have conflicting uses. (e.g. WordPress.com is using $_GET['theme_preview'] as well).

I'll create follow-up PRs to prevent that.

[okmttdhr]

CC: @scruffian @ajlende @draganescu
I've made two changes before this PR merged.

• Avoid changing non-theme-preview requests #51221 prevents Block Theme Previews from changing non-theme-preview requests.
• Also, I submitted [Block Theme Previews] Change the URL query string for more safety #51312 to change the URL query string name for more safety.

[scruffian]

I noticed that the button text for activating the theme has been modified and now doesn't make sense when you're previewing a theme:

It would be good to fix this before we merge this PR.

[scruffian]

Now #51361 is merged I think we can also bring this in.