Cleanup after [29179]:

* `_WP_Editors::editor_settings()` no longer needs to load MEjs styles
* Make sure each identical shortcode with multiple instances also has an iframe sandbox for each instance
* For the time being, make audio and video shortcodes bypass the loading placeholder to avoid whiplash visually

Props avryl, wonderboymusic.
See #28905.

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DB schema change: Add comment_author_email(10) index.

props tellyworth, pento.
fixes #21435.

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