Autosave: refactor autosave.js, use heartbeat for transport and move all...

Autosave: refactor autosave.js, use heartbeat for transport and move all "Add/Edit Post" related functionality to post.js. See #25272.

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git-svn-id: https://core.svn.wordpress.org/trunk@26872 1a063a9b-81f0-0310-95a4-ce76da25c4cd