oEmbed: Add support for the Vine endpoint.

props niallkennedy.
fixes #30426.

Built from https://develop.svn.wordpress.org/trunk@30503


git-svn-id: http://core.svn.wordpress.org/trunk@30492 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Sessions UI: Ensure screen readers provide feedback on action taken.

Also fixes some invalid HTML that occurs on DOM changes.

props joedolson.
fixes #30364.

Built from https://develop.svn.wordpress.org/trunk@30504


git-svn-id: http://core.svn.wordpress.org/trunk@30493 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Admin notices: Add more variety and make more generically usable.

There is now a base class of `notice`, with additional classes of `notice-success`, `notice-warning`, `notice-error`, and a new blue `notice-info`.

Also corrects some misleading notice colors, such as plugin tested up to warnings and login messages.

props avryl, melchoyce.
fixes #27418.

Built from https://develop.svn.wordpress.org/trunk@30505


git-svn-id: http://core.svn.wordpress.org/trunk@30494 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add support for `WP_Error` objects passed to `wp_send_json_error()`. The error object gets output as an array of error codes and messages, rather than as an empty object.

Fixes #28978
Props paulschreiber

Built from https://develop.svn.wordpress.org/trunk@30506


git-svn-id: http://core.svn.wordpress.org/trunk@30495 1a063a9b-81f0-0310-95a4-ce76da25c4cd