WordPress has supported custom page templates for over 12 years, allowing developers to create various layouts for specific pages.
While this feature is very helpful, it has always been limited to the 'page' post type and not was not available to other post types.
By opening up the page template functionality to all post types, we continue to improve the template hierarchy's flexibility.
In addition to the `Template Name` file header, the post types supported by a template can be specified using `Template Post Type: post, foo, bar`.
When at least one template exists for a post type, the 'Post Attributes' meta box will be displayed in the back end, without the need to add post type support for `'page-attributes'`. 'Post Attributes' can be customized per post type using the `'attributes'` label when registering a post type.
Props johnbillion, Mte90, dipesh.kakadiya, swissspidy.
Fixes#18375.
Built from https://develop.svn.wordpress.org/trunk@38951
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Boone Gorges
Pascal Birchler
Drew Jaynes
Scott Taylor