Delete expired transients on database upgrades.

Reverts [25416], which had all transients being cleared. This leaves much to be desired, but we don't want a core update to be blamed for breaking a site that incorrectly assumes transients aren't transient.

props dartiss, pento.
fixes #20316.

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Awesome about page design for WordPress 3.7.

The page includes a giant password meter that serves as both an animated example and a working demo. How cool is that?

props jorbin, melchoyce.
see #25603.

Built from https://develop.svn.wordpress.org/trunk@25839


git-svn-id: http://core.svn.wordpress.org/trunk@25751 1a063a9b-81f0-0310-95a4-ce76da25c4cd
Remove rarr from about page text. Use a subtle background instead.

props markjaquith.
see #25603.

Built from https://develop.svn.wordpress.org/trunk@25840


git-svn-id: http://core.svn.wordpress.org/trunk@25752 1a063a9b-81f0-0310-95a4-ce76da25c4cd