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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git-svn-id: http://core.svn.wordpress.org/trunk@28971 1a063a9b-81f0-0310-95a4-ce76da25c4cd