Introduce email templates for automatic background updates.

The four templates are:
 1. We successfully updated their site. If any of their themes or plugins are out of date, it also mentions that. ('success')
 2. We are not configured to update their site, so we notify them of the new release. ('notify')
 3. We tried but failed to update their site. The error was early in the process, so no harm, no foul. This is the same as template #2, plus one sentence. ('fail')
 4. We tried to update their site, and failed while copying files. ('critical')

With assistance from markjaquith.

see #10787.

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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.

Built from https://develop.svn.wordpress.org/trunk@25838


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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.

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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.

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