Add a filter for post_thumbnail_meta_box HTML

[coreygilmore]

It's been helpful to add additional HTML to the post thumbnail meta box, but without any hooks in post_thumbnail_meta_box​ the process is very fragile, and and requires the use of Javascript, or removing the postimagediv meta box and adding your own meta box.

_wp_post_thumbnail_html​ provides a way to filter the HTML, but function is only used for ajax requests.

The attached patch adds a filter to $thumbnail_html in post_thumbnail_meta_box, allowing for some parity.

I don't think _wp_post_thumbnail_html() fires at all anymore. Seems like we need backcompat for the admin_post_thumbnail_html filter, at least. Maybe for all of the output, even the JS, since it has title and update text in _e() and the filter is often used to change some of the text rather than using gettext().

Note that this will be fixed by the patch on #21776.

Duplicate of #21776.