Editor: when TinyMCE is used, add `autocomplete="off"` to the textarea to...

Editor: when TinyMCE is used, add `autocomplete="off"` to the textarea to prevent problems with missing <p> tags when the user reloads the page or uses Go Back/Go Forward buttons, and when editing the same post from two different windows. The autocomplete can be removed with the `the_editor` filter. Part props xsonic, fixes #27251.
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git-svn-id: https://core.svn.wordpress.org/trunk@27216 1a063a9b-81f0-0310-95a4-ce76da25c4cd