Improvements to the output of the new post navigation template functions, including swapping the position of the previous and next links.

See #29808
Props obenland

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Validate image data.

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Customizer: Use deep-links for Backgrounds, Headers, and Widgets.

Replace links in admin menu and toolbar to Custom Background/Header screen with deep-links to the Customizer section.
On the Widgets screen display a link to the Customizer widgets panel.

props topher1kenobe, rzen, celloexpressions, westonruter
fixes #25569, #25571, #28032.
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`grunt autoprefixer` for [30374].

see #29326.
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Simplify selectors in [30374].

props wonderboymusic, adamsilverstein.
see #29326.
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