Theme Installer: Revert to proxying through PHP for WordPress.org API requests.

This is to ensure we have valid installation nonces, though we've run into this as a problem previously (see #27639, #27581, #27055).

A tad slower, but we gained speed in 3.9 by simplifying the request made to the API.

props ocean90.
fixes #27798.

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About page design, first pass.

props melchoyce, ryelle.
see #27713.

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Inline documentation for hooks in wp-includes/media.php.

Props scottlee for the initial patch. Props kpdesign, DrewAPicture.
Fixes #27718.

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Add a TTL to core update checks to allow us to narrow the 12-hour update window.

props dd32.
fixes #27772.

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Inline documentation for hooks in wp-includes/general-template.php.

Props jesin, kpdesign.
See #27719.

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Inline documentation for three remaining hooks in wp-includes/general-template.php.

Documents the `login_form_top`, `login_form_middle`, and `login_form_bottom` and filters.

That's 184/184 hook docs files complete. Boom!

Props kpdesign, DrewAPicture.
Fixes #27719.

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