Deprecate `format_to_post()` and remove its filter. Fixes #25526. Props simonwheatley
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Admin Menu: Use `.dashicons-before` as default $img_class.

props kovshenin for noticing and testing.
see #26630.
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TinyMCE: when parsing [caption] and the width attribute is missing, recreate it from the image tag width, fixes #23103
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TinyMCE: remove border-radius and padding from placeholder images, props avryl, see #27106
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TinyMCE: add the <hr> plugin and button, see #27159
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Add has-post-thumbnail as a post class.

props danielbachhuber, frank-klein.
fixes #18804.

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