Always convert auto-drafts to drafts in edit_draft() and _wp_translate_postdata().

This regressed due to refactoring in [26995]. This commit fixes Quick Draft.

see #25272.

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Multisite load: Properly call wp_cache_set().

props gradyetc.
see #27003.

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Deprecate get_current_site_name(). see #27003.

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Update mce-view.js and the `wpview` TinyMCE plugin, and use them to show gallery previews in the Visual editor, props gcorne, see #26959
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