Clarify the behaviour of the `wp_editor_set_quality` and `jpeg_quality` filters.

Merges [30874] to the 4.1 branch.

Props DH-Shredder
See #29856

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Twenty Fifteen: remove license details for the image shown in the theme  screenshot file as it is Creative Commons Zero, which doesn't require attribution.

Merges [30867] to the 4.1 branch.

Props iamtakashi
See #30156

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Fix search box position on the Add Plugins screen after resizing the window to narrower than 1000px and then back to wider than 1200px.

Merges [30830] to the 4.1 branch.

Fixes #30658.

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Correct the height of the search type `select` element when searching the plugin directory.

Merges [30879] to the 4.1 branch.

Fixes #30719
Props pavelevap, eliorivero

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Twenty Fifteen: remove fonts as a style dependency.

Merges [30832] to the 4.1 branch.

Props obenland, SergeyBiryukov.
Fixes #30678.

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Twenty Fifteen: remove Genericons as a dependency from loading the main stylesheet.

Merges [30865] to the 4.1 branch.

Props iamtakashi
See #30532.

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Twenty Fourteen: remove Genericons as a dependency from loading the main stylesheet and IE-specific stylesheet.

Merges [30866] to the 4.1 branch.

See #30532.

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