TinyMCE: add the <hr> plugin and button, see #27159
Built from https://develop.svn.wordpress.org/trunk@27428


git-svn-id: http://core.svn.wordpress.org/trunk@27275 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Add has-post-thumbnail as a post class.

props danielbachhuber, frank-klein.
fixes #18804.

Built from https://develop.svn.wordpress.org/trunk@27429


git-svn-id: http://core.svn.wordpress.org/trunk@27276 1a063a9b-81f0-0310-95a4-ce76da25c4cd