Add imgur to the list of oEmbed providers.

props bradparbs.
fixes #21632.

Built from https://develop.svn.wordpress.org/trunk@27113


git-svn-id: http://core.svn.wordpress.org/trunk@26980 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Revisions browser: The back button should return to the post you were editing (use replaceState instead of pushState).

props adamsilverstein.
fixes #26997.

Built from https://develop.svn.wordpress.org/trunk@27114


git-svn-id: http://core.svn.wordpress.org/trunk@26981 1a063a9b-81f0-0310-95a4-ce76da25c4cd