Update MediaElement.js to a new 2.12.1 (untagged) build. see #24183.



git-svn-id: https://core.svn.wordpress.org/trunk@24786 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Autosave: Don't save an autosave unnecessarily when the post editor loads and TinyMCE is the default editor.

Delete such useless autosaves when we detect them in edit-form-advanced.php.

see #7392.



git-svn-id: https://core.svn.wordpress.org/trunk@24787 1a063a9b-81f0-0310-95a4-ce76da25c4cd