Responsive embeds is a way for a theme to opt in to WordPress dynamically scaling the width/height of an embed. When a theme supports responsive embeds, a `wp-embed-responsive` class is added to the `<body>` tag. This information is also presented through the REST API for clients to respect.
Merges [43790] and [43791] from the 5.0 branch to trunk.
Props desrosj, danielbachhuber, ocean90.
Fixes#45125.
git-svn-id: https://develop.svn.wordpress.org/trunk@44138 602fd350-edb4-49c9-b593-d223f7449a82