Improve Sandboxing

[westonruter]

Summary

This is a follow-up to #6546. Namely, it seeks to address key Future Todos.

Changes

• Move "Sandboxing Level" setting from being inside Standard mode drawer to being its own drawer among the Advanced Settings. The drawer is only displayed if the experiment is enabled and if Standard mode is selected.
• Prevent calling Sandboxing::finalize_document() if the experiment is not enabled. See Experimental: Add sandboxing levels #6546 (comment).
• Omit AMP runtime script (v0.js and v0.mjs), AMP runtime style, and AMP CDN preconnect link if they are not needed due to no other AMP scripts being on the page and valid AMP is not the target (level 3).
• Refactor threaded comment handling to remove an erroneously-enqueued comment-reply script when there isn't a comment form on the page. (Hestia is enqueueing this script whenever is_singular() and isn't checking if comments_open() && get_option( 'thread_comments' ).)

Before	After

Checklist

• My code is tested and passes existing tests.
• My code follows the Engineering Guidelines (updates are often made to the guidelines, check it out periodically).

[github-actions]

Plugin builds for 61ec39d are ready 🛎️!

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[dhaval-parekh]

Changes look good to me. 👍

[milindmore22]

LGTM 👍🏼

[milindmore22]

QA Passed

Sanboxing settings are moved to the new drawer, AMP sandboxing levels are working as expected.