* Prevent loading `wp-embed` script unconditionally on every page in favor of conditionally enqueueing when a post embed is detected. The `wp-embed` script is also explicitly marked as being in the footer group. Sites which currently disable post embed scripts from being enqueued via `remove_action( 'wp_head', 'wp_oembed_add_host_js' )` will continue to do so.
* Send a `ready` message from the host page to each post embed window in case the `iframe` loads before the `wp-embed` script does. When the `ready` message is received by the post embed window, it sends the same `height` message as it sends when it loads.
* Eliminate use of `grunt-include` to inject emoji script and the post embed script. Instead obtain the script contents via `file_get_contents()` (as is done elsewhere in core) and utilize `wp_print_inline_script_tag()`/`wp_get_inline_script_tag()` to construct out the script. This simplifies the logic and allows the running of src without `SCRIPT_DEBUG` enabled.
* For the embed code that users are provided to copy for embedding outside of WP, add the `secret` on the `blockquote` and `iframe`. This ensures the `blockquote` will be hidden when the `iframe` loads. The embed code in question is accessed here via `get_post_embed_html()`.
Props westonruter, swissspidy, pento, flixos90, ocean90.
Fixes#44632, #44306.
Built from https://develop.svn.wordpress.org/trunk@52132
git-svn-id: http://core.svn.wordpress.org/trunk@51724 1a063a9b-81f0-0310-95a4-ce76da25c4cd
9038612705