Correct a missing conditional in `edit-form-advanced.php`.

See #26631, [27209]. 
Props nacin for noticing.


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Add boolean return values to require_if_theme_supports().

props jeffsebring, DrewAPicture.
fixes #26716.
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Prevent "Add" button in Tags meta box from wrapping to a new line in ru_RU.

fixes #26510.
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